Are you taking about changing the pack itself? You could take a 7 cell pack and snip one cell out of it and re-solder the connector back on without a problem, but if you add a new cell to your existing pack, you would be asking for trouble. Otherwise you are just changing what pack you use, you are just talking weight/voltage. There are so many combinations that could happen with this, its hard to give you any concrete advice. First your motor is limited by how many volt or amps it can handle. What motor or gear box are you using? Next is what packs are you thinking of? If you try to draw more amps than the pack is capable of, it will start shedding volts. You can also change your prop/gearbox combination to effect how many amps you draw depending on the pack you want to try. And, just to confuse you a little bit more, do you want a slow flyer with lots of thrust or a fast plane the doesn't have a ton of vertical. (I know you want both right )