I believe it's made on a modified version of hpl 2 - hpl 2.5 so to speak
I also think I remember Samuel stating in the discussion thread that they were unable to open maps from TDD with the AMFP version due to some code changes.
So be wary, porting your cs over might be not as easy as it seems. But then, why would you need to? You can probably still use the new models should you need them (either by using the .ent files or by creating new ones with the .dae files) and it's not like the community for TDD cs's will suddenly disappear when the new game comes out