Hi Adam,
Do you mean you want to install Office 365 on your new Mac? As far as I know, you can install Office 365 on up to five Mac or PC computers. So, you don’t need to change subscription from PC to Mac.
For your reference:
http://office.microsoft.com/en-us/support/install-office-on-more-than-one-computer-HA102901468.aspx
If there is any misunderstanding, please let me know.
Thanks,
Dunk Shen