Technically, tickets are supposed to be excluded, but empirically, it seems that if your tickets are cheap enough (likely needs to be under $100 and possibly even needs to be under $50), lots of people report getting reimbursed. Here's my data point, but it's been nearly a year and a half since I did it: https://www.flyertalk.com/forum/33471327-post441.html

Another way that should work if you're okay with wallet credit would be to make a non-Saver booking, then in a separate transaction, pay for an upgrade to PC. You may or may not be automatically reimbursed, but if not, since seat selection is definitely valid for reimbursement, you can call or chat and request a reimbursement. Just mention that you want to be reimbursed for "seat selection" and there shouldn't be an issue if you say those magic words. Once the reimbursement posts to your account, cancel your flight for wallet credit, and the cost of the flight plus the PC upgrade should go to your wallet.