Renaming the split archive's parts does not corrupt the contents. As long as the parts are sequentially named you should have no problems extracting either.
I just tested this in fact with 7zip. Created a split archive with parts test.zip.001 - test.zip.005, which I was able to extract fine. Renamed to best.zip.001 - best.zip.005 and it extracted fine. Renamed just the last part to best.z05 and on extracting, got a single best.zip just as you described. Extracting this archive threw an invalid archive error, again matching your experience.
If you're using Windows and have file extensions hidden (as they are by default), it's easy to have thought you renamed the files properly but you might have ended up with mixed extensions. Renaming all the parts in the exact same recommended format i.e. name.zip.001 - name.zip.NNN should fix this.