DL switched from using W fares to P/A/G fares for Premium Economy fares on VS not too long ago. They also got rid of S coach fares. It just looks like they haven't bothered to update the website as it still lists W and S fares as eligible fare classes. So I would expect your fare to be eligible even though the website does not list it.
But because the website does not list it, you may have trouble finding an agent who agrees that conceptually it should be eligible despite the website not listing it as such. Might have to call back a couple times until you find a particularly helpful agent, and may require them escalating to a particularly thoughtful supervisor. Good luck.
Not clear if you're trying to use GUCs or miles to upgrade, but you actually may have better luck trying to upgrade with miles -- that program is newer and I believe was specifically set up to incorporate the possibility of upgrading from PE to UC. Even if you'd prefer to use GUCs it could be worth checking how many miles they'd charge to upgrade, as it may be a relatively small amount given you're not going all the way from Y to UC.