Well, unfortunately a freeze is a one way operation so you're more than a little stuck in regards to a straight undo.
However, if you look on the construct tab in Modeller, you will find a number of tools to help reduce the polygon count namely:
- Merge Polygons
- Reduce Polygons
- quemLOSS2
You'll need to refer to the LW docs and resort to a little trial and error in order to obtain the best results (quemmLOSS2 can be particularly destructive).
A quick search in the flay plugins database using the keyword 'reduce' also turned up the following:
- Decimate 2
- quemLOSS3
- Polygon Cruncher
Hope this helps...