Licensing question, so Tech Support it is.
With that said, note that the licenses are stand-alone export licenses. Console licenses do
not grant you everything a Desktop license grants you. They'll allow you to export to the specific console and nothing more. If you still want to export to Desktop targets, you'll need the Desktop license as well.
For this reason, there is no such "upgrade" discount, as you're not "upgrading" licenses, but buying a separate one. The only licenses that could be considered an "upgrade" in terms of functionality are the Ultimate license, as this grants you access to all functionality, including exports, for a year, as well as the Desktop license, which can be considered an upgrade of the Creator license.
As of writing, the only upgrade path that exists is the Creator license, which gives you a discount on the Desktop license.
Speaking of console exports, please note that everything I said above assumes that you fulfill the third-party requirements (which are not imposed by YoYo Games) for using them. You'll have to be a registered developer with the manufacturer of the console(s) of your choice before you'll even be allowed to purchase a console export.
So, to answer what remains of this question: What you
can do is start developing a project now, using whatever export licenses you have or will purchase, and then purchase a console license later, adapt anything that requires platform-specific modifications, and otherwise use the same code base.