Tech support basically told me the same thing...cannot be done. I called the License group and explained the situation and they sent me another license. I placed it in the same folder with the inital license. I don't know where my IT team got the off-the-network ID from; I'll have to ask. There were so many attempts to correct this that I'm concerned that I missed some crucial step. Again, I'll follow up with my IT team to get more details.
This whole event took a week or so to get sorted out. A real pain. I had issues installing 15 too. Don't use the "setup" icon in the zip file.
I'll follow up next week.