By all mean, YES. That’s the best time to do it.
I’m so glad you’re asking about that now, before you’ve bought the upgrade.
Too often we hear from people who have already used their authorization code to create a username and serial number for a particular OS platform (Win or Mac) and then decide later they’ve changed their minds. Because of the way we do things, the license can only work on one platform. We can’t change it after the fact - so changing platforms when you’ re upgrading is the thing to do!