Published upgrade price for a given year/version doesn't specify Mac/Windows, so I'm guessing you'd get a discount even if cross-grading from Win-to-Mac or vice versa.

The way to know officially is a quick call or Chat (top of page) with PGMusic staff.

Offers change all the time, so I was going from the published pricing chart at the following link. If I know PGMusic support, they'll honor a published discount or suggest a better one.
http://www.pgmusic.com/bbmac.packages.htm

Personally I've had really good luck with Chat .. someone always seems to respond fairly quickly.
YMMV, but I'd start there. You may have an official best answer/price/offer within minutes.

I do not work here, but the benefits are still awesome
Make your sound your own!