I seem to have got through the first 14 replacements and then hit a brick wall, but I had no problem with the first across replacement. I made the obvious change in 13a, then changed 2d, and so the next across was 22a. I changed the first letter and then there was a perfectly sensible alteration for 11d. In each case it was possible to find three successive letters in the clue which contained both replacements.
Presumably the final step will involve another replacement in 13a. I am not sure what that is going to give me for the final version of this entry.