Half of my music collection was purchased with an old Apple ID, using a defunct email address. I can play music from both accounts on my devices, no problem. But when iCloud comes out, I wonder how I'll be able to have it sync both accounts as one cloud in the iCloud? I wish I could merge accounts and be done with the old one.
Best to you and your hat maker,
Your loving nephew,
This is where iTunes Match really can jump in and make your life easier -- with one small catch. For US$25, it gives you access to all your music. If your music is not DRM protected, you can just throw it into your normal library and Match will enable it in the cloud.
The problem comes in with older purchases made before the iTunes Plus switch away from DRM. Auntie isn't sure that music from your older account will transfer over to the cloud -- and Apple hasn't clarified how iTunes will treat music that's authorized to play on your computer but DRM'ed to another account.
Unfortunately, Apple does not allow you to merge accounts. They don't do it for partners getting married; they don't do it for transfer from MobileMe; they just don't do it. You are not alone in this -- Apple has resisted account merging for years.
So as far as Auntie can tell, your only problems may arise with those older purchases. Otherwise, iTunes should not be able to differentiate between music copied from your old account and music purchased or ripped in your new one. (And if it does, there are any number of metadata stripping utilities you can google for.)
Hugs and good luck,