Oh dear. It seems there is an ambiguity then. I have an answer for one of the final transformed entries that is definitely correct and is not in Chambers.
There are many thousands of candidates for the modified grid entries so I guess this is a potential problem that the setter and editors may have not been able to exclude.
My non Chambers option is the most southeasterly in the grid. I have not changed the first letter but all others are different.
P.S. There is a very similar word in Chambers but with a different last letter which gives real real words but it is not really an acceptable thematic option.