Thanks Mike and Jeff.
First it says you must give up all licensing rights to the platform you are transferring from.
The swap does not help me as I have 2 Macs,one in the studio and one in the office and cannot give up either.
I guess that means I really need the third license for the Pc.
You can install a PS licence on two machines, so if you already have two licences, then one will cover your Macs. The other after converting, will do your PC.