GregM Ummm...that's a novel way of blocking out the trem system (Setting it to a fixed position). I'm a fan if you don't plan on using the trem, though that method is about as Junkie as I've seen 😆
Usually peeps either set the bridge (strat style w/2 post) flat against the body or use a block of wood in a floyd rose style trem to keep it in a fixed position. I guess wood was in short supply 😁 Likely if that's how they blocked out the trem, what other decisions did they make?
That said, what can you do from here - depends on how much guitar tech skills you have/want to develop. Easy answer, take it to a decent tech.
If a tech is not a option - Changing string gauges and tuning does mean re-visiting the setup. WIthout seeing the guitar it's tough to diagnose. I suspect your neck angle is choking the notes > 12th fret. The fix? Read up on shimming a neck - is a art form as it's one part of playability (relief (truss rod adjustment), action (saddle & nut adjustment)). But not for the faint hearted, it takes me multiple attempts to dial it in usually.
There might be something odd going on, perhaps a fret is slightly higher than the rest or there's a bow/twist to the neck (it happens). Also heavier strings means the nut slots might need deepening, though that's not contributing to your issue as described
Good luck!
