I've read somewhere (unfortunately I can't recall where) that they tried telescopic schnorkels on some of the conventional U-boats by removing one of the periscopes and installing it there. However I don't know if this was the case with the planned production version of the VIIC/42.:hmm:
Edit: Here they mention it in a few lines but not which boats that where rebuilt.
http://www.uboataces.com/snorkel.shtml