as far as the tube is concerned, there is no difference between +500v on the plate and a grounded cathode, and a grounded plate and -500v on the cathode. Either way the tube only sees it has a plate 500v more positive than its cathode.
Would that be unconventional? Sure, but otherwise not an issue.
Imagine a TTL chip, usually +5 on the top and ground on the bottom, but just as happy with ground on top and -5 on the bottom. We have some amps that have op amps running between ground and -30v instead of between +/-15v.
If the customer used to be happy with it, are we sure he will be happy if we "fix" this issue?