All that downgrading your PSP consists of is tricking your PSP into "upgrading" to version 1.5. So, if you get a safe downgrader, then there's no reason you can't bounce back and forth as many times as you want to. All that's involved upgrading to 2.0 is using a firmware 2.0 installer.
Here's the caviat: some PSP downgraders have been known to "brick" (render un-bootable, where the PSP won't start at all) PSPs. I understand that these downgraders are malicious (someone's trying to kill PSPs, it's not a bug) and that they are few and far between. That being said, it's a risk.
I got my downgrader from a private BT tracker, but a good site for PSP downloads of all kinds is
PSPUpdatates.qj.net.
P.S. It's possible that you can use this:
WAB Version Changer V2.5 to play your 2.0 apps. it's at least worth a try.