If you used a PC last year and want to switch to the Mac version this year (or vice-versa), you are still eligible for the discounted update.
In fact, when you order we give you BOTH the PC & Mac versions. So just go ahead and order the discounted update, and your account will be given access to both versions.
Just make sure to login through your FSM Account to be able to access the discounted rate: https://www.fansoftmedia.com/store/member
If you want to import your league settings, the xxx.fan league files are in fact compatible with both Mac and PC, so just transfer your league file onto your new system.