diff --git a/panda/src/gobj/geomVertexColumn.I b/panda/src/gobj/geomVertexColumn.I
index b7d585f092..e74e47d7d2 100644
--- a/panda/src/gobj/geomVertexColumn.I
+++ b/panda/src/gobj/geomVertexColumn.I
@@ -16,7 +16,7 @@
 ////////////////////////////////////////////////////////////////////
 //     Function: GeomVertexColumn::Default Constructor
 //       Access: Private
-//  Description: Creates an invalid column.  Used on when constructing
+//  Description: Creates an invalid column.  Used only when constructing
 //               from a bam file.
 ////////////////////////////////////////////////////////////////////
 INLINE GeomVertexColumn::
diff --git a/panda/src/gobj/geomVertexColumn.cxx b/panda/src/gobj/geomVertexColumn.cxx
index 5668dd7b98..0c272ad477 100644
--- a/panda/src/gobj/geomVertexColumn.cxx
+++ b/panda/src/gobj/geomVertexColumn.cxx
@@ -31,9 +31,84 @@ operator = (const GeomVertexColumn &copy) {
   _start = copy._start;
   _column_alignment = copy._column_alignment;
 
-  delete _packer;
-  _packer = NULL;
+  setup();
+}
 
+////////////////////////////////////////////////////////////////////
+//     Function: GeomVertexColumn::set_name
+//       Access: Published
+//  Description: Replaces the name of an existing column.  This is
+//               only legal on an unregistered format (i.e. when
+//               constructing the format initially).
+////////////////////////////////////////////////////////////////////
+void GeomVertexColumn::
+set_name(InternalName *name) {
+  _name = name;
+  setup();
+}
+
+////////////////////////////////////////////////////////////////////
+//     Function: GeomVertexColumn::set_num_components
+//       Access: Published
+//  Description: Changes the number of components of an existing
+//               column.  This is only legal on an unregistered format
+//               (i.e. when constructing the format initially).
+////////////////////////////////////////////////////////////////////
+void GeomVertexColumn::
+set_num_components(int num_components) {
+  _num_components = num_components;
+  setup();
+}
+
+////////////////////////////////////////////////////////////////////
+//     Function: GeomVertexColumn::set_numeric_type
+//       Access: Published
+//  Description: Changes the numeric type an existing column.  This is
+//               only legal on an unregistered format (i.e. when
+//               constructing the format initially).
+////////////////////////////////////////////////////////////////////
+void GeomVertexColumn::
+set_numeric_type(NumericType numeric_type) {
+  _numeric_type = numeric_type;
+  setup();
+}
+
+////////////////////////////////////////////////////////////////////
+//     Function: GeomVertexColumn::set_contents
+//       Access: Published
+//  Description: Changes the semantic meaning of an existing column.
+//               This is only legal on an unregistered format
+//               (i.e. when constructing the format initially).
+////////////////////////////////////////////////////////////////////
+void GeomVertexColumn::
+set_contents(Contents contents) {
+  _contents = contents;
+  setup();
+}
+
+////////////////////////////////////////////////////////////////////
+//     Function: GeomVertexColumn::set_start
+//       Access: Published
+//  Description: Changes the start byte of an existing column.
+//               This is only legal on an unregistered format
+//               (i.e. when constructing the format initially).
+////////////////////////////////////////////////////////////////////
+void GeomVertexColumn::
+set_start(int start) {
+  _start = start;
+  setup();
+}
+
+////////////////////////////////////////////////////////////////////
+//     Function: GeomVertexColumn::set_column_alignment
+//       Access: Published
+//  Description: Changes the column alignment of an existing column.
+//               This is only legal on an unregistered format
+//               (i.e. when constructing the format initially).
+////////////////////////////////////////////////////////////////////
+void GeomVertexColumn::
+set_column_alignment(int column_alignment) {
+  _column_alignment = column_alignment;
   setup();
 }
 
@@ -146,6 +221,10 @@ setup() {
 
   _total_bytes = _component_bytes * _num_components;
 
+  if (_packer != NULL) {
+    delete _packer;
+  }
+  
   _packer = make_packer();
   _packer->_column = this;
 }
diff --git a/panda/src/gobj/geomVertexColumn.h b/panda/src/gobj/geomVertexColumn.h
index 1794ab3d9a..09510a3e89 100644
--- a/panda/src/gobj/geomVertexColumn.h
+++ b/panda/src/gobj/geomVertexColumn.h
@@ -63,6 +63,13 @@ PUBLISHED:
   INLINE bool overlaps_with(int start_byte, int num_bytes) const;
   INLINE bool is_bytewise_equivalent(const GeomVertexColumn &other) const;
 
+  void set_name(InternalName *name);
+  void set_num_components(int num_components);
+  void set_numeric_type(NumericType numeric_type);
+  void set_contents(Contents contents);
+  void set_start(int start);
+  void set_column_alignment(int column_alignment);
+
   void output(ostream &out) const;
 
 public:
diff --git a/panda/src/gobj/geomVertexData.cxx b/panda/src/gobj/geomVertexData.cxx
index 0da6a17266..b2fee3e7c0 100644
--- a/panda/src/gobj/geomVertexData.cxx
+++ b/panda/src/gobj/geomVertexData.cxx
@@ -331,6 +331,48 @@ set_format(const GeomVertexFormat *format) {
   cdataw->_animated_vertices.clear();
 }
 
+////////////////////////////////////////////////////////////////////
+//     Function: GeomVertexData::unclean_set_format
+//       Access: Published
+//  Description: Changes the format of the vertex data, without
+//               reformatting the data to match.  The data is exactly
+//               the same after this operation, but will be
+//               reinterpreted according to the new format.  This
+//               assumes that the new format is fundamentally
+//               compatible with the old format; in particular, it
+//               must have the same number of arrays with the same
+//               stride in each one.  No checking is performed that
+//               the data remains sensible.
+////////////////////////////////////////////////////////////////////
+void GeomVertexData::
+unclean_set_format(const GeomVertexFormat *format) {
+  Thread *current_thread = Thread::get_current_thread();
+  nassertv(format->is_registered());
+
+  CDLockedReader cdata(_cycler, current_thread);
+
+  if (format == cdata->_format) {
+    // Trivially no-op.
+    return;
+  }
+
+#ifndef NDEBUG
+  nassertv(format->get_num_arrays() == cdata->_format->get_num_arrays());
+  for (int ai = 0; ai < format->get_num_arrays(); ++ai) {
+    nassertv(format->get_array(ai)->get_stride() == cdata->_format->get_array(ai)->get_stride());
+  }
+#endif  // NDEBUG
+
+  CDWriter cdataw(_cycler, cdata, true);
+
+  // Assign the new format.
+  cdataw->_format = format;
+
+  clear_cache_stage();
+  cdataw->_modified = Geom::get_next_modified();
+  cdataw->_animated_vertices.clear();
+}
+
 ////////////////////////////////////////////////////////////////////
 //     Function: GeomVertexData::clear_rows
 //       Access: Published
diff --git a/panda/src/gobj/geomVertexData.h b/panda/src/gobj/geomVertexData.h
index 335e834e84..152446878d 100644
--- a/panda/src/gobj/geomVertexData.h
+++ b/panda/src/gobj/geomVertexData.h
@@ -100,6 +100,7 @@ PUBLISHED:
 
   INLINE const GeomVertexFormat *get_format() const;
   void set_format(const GeomVertexFormat *format);
+  void unclean_set_format(const GeomVertexFormat *format);
 
   INLINE bool has_column(const InternalName *name) const;