Hello,
Plesk Linux and Plesk Windows are to different licenses, with two different license keys. If we would "switch", you would have two licenses and paid for only one.
There is also a big note online about that fact:
http://www.swsoft.com/en/buyonline/plesk82/
"NOTE: Plesk 8.2 for Linux/Unix license purchases are only for use on Plesk (8.0 - 8.2) for Linux/Unix servers and are not able to be used or transferred to Plesk for Windows servers."
Best regards,
Peter Domke
Manager Hosting Sales