Andy, if you aren't sure about the battery, get a load test done. If that shows that the battery is OK, I would keep it until you develop starting problems. I would choose a battery for its cold cranking capacity, not it's summer reserve capacity. (That of course depends on where you live). In cold months batteries are their weakest. Regarding air conditioning, if your alternator is working properly, a larger battery in the summer months wouldn't really be necessary, IMHO. I find that most batteries are junked because of winter starting problems.

Gary