In your situation I'd just install the old app. Play with it long enough to find out if it works for you. Then sometime later decide if you want to upgrade the whole phone.
Upgrading would be a moderate hassle. You'd have find the rom, download it, find out what's the procedure for installing over an existing install, and your phone has to be rooted already. I'd be expecting most if not all of your apps to keep working. And you'd probably want to find out how to back up your existing installation first, and not just the data but also the actual OS...
I have experimented briefly with Android 4 on my HTC Desire, but I'm sticking with 2.3.7 because basically my phone doesn't have the grunt to support the software (and the current roms for the Desire are too unfinished to be properly useable). If I had an S2, though.... There's an official Android 4 update for that phone, meaning presumably all the bells and whistles are present and the hardware is actually capable of running them, and even maybe the official rom can be found online somewhere for download to install by yourself. I'd do it to see how it worked. But that kind of thing is fun for me.
So, short answer: see what the old app looks like. It might end up being good enough for your purposes.