If I install one from, say, 10.2012 will it fail to show my transactions since then? what other problems will it cause?
The wallet contains keys in a key pool. So a backup taken today will already include the keys for my next 100 transactions where a new address is generated for change (or where an address is generated because I clicked New).
So any recent backup probably has all the keys you've used ... unless you do a lot of SatoshiDICE wagers or that kind of activity.
Wallets can be restored, then another one restored without any impact.
You will want to launch the client with -rescan though the first time after each restored wallet.dat.