As worded it sounds like the sepll/ability will end up targetting both the equipped creature AND the new target you choose. Unless this was the idea, this would read better as "Whenever equipped creature would become the target of a spell or ability, you choose a different target instead." Like the concept.

Thanks!

This current wording is somewhat confused between being a replacement and a triggered ability. I'd make it triggered, and say "Whenever equipped creature becomes the target of a spell or ability, change the targets of that spell or ability." If you don't want the controller to be able to Giant Growth it, that'd get more fiddly and say something like "...change the targets of that spell or ability to targets other than ~."