what happens is: between the bridge and the neck there is X amount of tension which is provided by how tightly tuned the strings are... when you put thicker guaged strings on, it takes more tension to bring these thicker strings to the same note as it would for lighter strings... therein, you're changing the amount of pull on the bridge/tremolo. this actually will pull the bridge forward and throw everything out of whack... now, if you're going up on size... say, .11's to .12's it shouldn't make a HUGE difference... but if you're going from a set of .10's up to a set of Ernie Ball Slinky top Heavy Bottom .11's... where the bass strings are very thick... you're going to need to tension.
so, if you're planning on going up to bigger guage strings i would suggest adding 2 more springs. chances are your guitar came stock with only 3 springs... i always use 5 trem springs becuase i don't use the trem often and it helps with tuning stability.
i would suggest that if you're planning on doing this you just take it into the guitar shop when you buy the strings and have the store do it if you're not comfortable...... also, when yo're making a large string change you can throw the intonation off as well... another reason to take it to a shop.
