This doesn't work at all. The only entry that gives the required thematic name by following the thematic process is the five-letter entry on the right of the grid (crossing 25 & 26). The others are just completely wrong and follow a pattern that seems to be of the setter's devising. At the very least the preamble should have referred to the modified application of a process, etc.
The definition for 11a is very misleading. It is the precise definition given for TRANSMIGRATING, and for a while I thought PIG, MIG, RAT were relevant. The actual word needed has a slightly differently worded definition. Normally that wouldn't matter, but in the absence of any word length given for the longer words it's very significant.
It's a poorly devised puzzle in my view.