Yesterday I replaced the Elixir light strings (12-53) on my Yamaha FG830 dread with a set of Elixir Light-Medium (12-56). I immediately noticed that... the action changed ! Instead of being the perfect 3/32 and 2/32 that the guitar always had with light strings after I did a set up, the action went up to 4/32 and 3/32 ...
I checked the neck relief and it was perfectly fine, almost flat as I like it. So... I am trying to understand why the action changed. I thought about two possible reasons: 1) the increased tension is pushing the top (and consequently the bridge) higher therefore increasing the height of the strings. 2) Due to increased tension, the neck angle slightly changed, therefore increasing the height of the strings.
Any opinion ? Is this normal ? Also, should I be worried and go back to light gauge strings ?
Thanks and good luck!
The neck got out of line, turned up from the neck/body joint or the bridge pulled up. Hard to imagine on a robust guitar like an 830. Bring the ole straight edge and see what happened.
Hmmmmmmmmmmm. The only change that going up in string gauge by that small amount should make is in the neck relief - and even that should be minimal if it happens at all. The actual "action" shouldn't have changed.

I think Tony's advice of going back to lights is the best thing to do - personally I think standard 12 gauge is as high as you should go anyway.