Hey Harry. It shouldn’t; you don’t actually uninstall core to activate PRO, so those will remain in place and unmodified. For the new views on the frontend – recurrence or venue, for example – you may need to add customizations (as those views weren’t present in the version you modified). But otherwise it should be consistent.
I’d backup first as a matter of general safety, though. Hope that helps and please let me know if you need anything else here. Thanks for your interest.