Credit that is still associated with a ticket on AS stock (027) can be used towards the purchase of any other ticket on AS stock (027), including those involving codeshares. So when you buy a ticket but don't use it, you can retain the ticket number and later change it to one that involves DL or AA.

Credit that has lost all association with its original ticket (like credit certificates or value in your My Wallet) can only be used towards flights marketed, sold, and operated by AS/QX. Basically once you turn a ticket into raw "Alaska Dollars," it can only be spent on flights operated by AS/QX.

I don't know all the reasons behind this logic (probably has to do with the agreements AS has in place with partners to calculate who gets what share of the ticket price), but that's the way it works. If you want to retain value towards future flights involving codeshares, don't deposit credit in your My Wallet. Instead, just keep the old ticket numbers around.

(I think the one other subtlety is that old tickets can only be exchanged 1-for-1 for new tickets. So you can't combine two cheap tickets into one expensive ticket. For that you have to use certs or My Wallet, which means only AS/QX-operated flights.)