You should be able to reissue the license inside your WHMCS for the addon in the future. WHMCS team is also very flexible, so I'm pretty sure they would have no problem switching it to your own license later on. But double check with them just to be sure.
It would be best if you contact whmcs directly and explain your question to them. This you can get a exact answer from there and verify that your plan will work out . There's plenty of times in the past I buy something to use it in the future with something else like you state above and it does not work out.
I'm not saying anyone here is wrong or right but going through whmcs.com the actual defelopersthen you'll get a much more better answer.