I recently bought a Yamaha 6000 Acoustic. Then after some time I changed the string gauge from 10 to 12. The strings were really high after that, the strings wouldn't stay in standard tuning due to tension so after some relief with truss rod adjustment, the strings were even more higher.
I asked a tech at a local music store and he said you have to tighten the truss rod to lessen the height but that wouldn't allow the strings to be in standard tune due to tension. I suggested to sand down the saddle, he said there would be no guarantee of intonation after that. What should I do?
Sand down the saddle. The tech is full of shit. The guitar's intonation will not be significantly affected by lowering the action. They ship guitars high on purpose. Just make sure there is a healthy amount of saddle above the bridge before you start. If there isn't, and the action is too high, the neck is set wrong.

Here's a guide to teach you how to set u an acoustic guitar: http://thbecker.net/guitar_playing/guitars_and_setup/setup_page_01.html

You are correct in assuming there needs to be a certain amount of relief in the neck.

The neck relief needs to be set before you adjust the string height. The truss rod adjustment, (loosening), to provide relief will cause the action to rise. BUT, beyond that, you never use the truss rod to set action height, period.
Last edited by Captaincranky at Aug 8, 2013,
i'll go with... " can i just put the 10's back on?" Alex
that's not a very high end guitar and the string tension differential is drastic for that thing.
need more gear and a lot more talent(courtesytuxs)