oldwave,
Technically, no you don't need to upgrade the license to upgrade the system.
The issue you are going to have is that the license can be 'lost' during the upgrade process and you would need a copy of it.
The copy you would have to get from the VAR who owns the Licensing for that system OR you may be able to contact Mitel and get it transferred to you if you are a certified VAR.
Before you ask, no there isn't a way to download the license from the system, sorry.
Thanks,
TE