Thanks for the clarity!
After receiving some very helpful offline assistance, this is how it turned out (definitely more to do with details):
1. Validation errors were found and corrected. There was a discrepancy between the domain assigned to a field in the main feature class and then within Subtypes, if that makes any sense. One was Yes_No while the other was 1_0. For some reason the version 7 check-out tool seemed to let it go but the version 10 one caught it.
2. Re. the custom form, my VBscript referred to data tables by name that are generated as part of the axf. It seems that the version 7 check-out tool and the v10 tool create tables using slightly different naming conventions, and in one case the v10 tool split the data into 2 tables where the v7 tool only generated one. It was a slightly laborious but straightforward task to correct this and get it working.
Very glad the forum was there to help with my newbie issues.