installation of Pro and Personal?

I still have supporting projects related to 5.6.x Pro, yet am needing to get up on changes with 2018.x, but I can only use personal with it currently.

It seems to not like the fact that my Pro account/serial is registered and has required me to basically create a new account (or in my case use a really... really old one from like 4.2 days when I first got started with Unity). And run Unity with it.. which is fine and all, but a bit of a hassle as now 5.6.x lost it's registration and I'm guessing I will be in the logout/logon license input cycle for a ... long time?

Was there something I missed somewhere about this?.. obviously, but even multi-account support per machine should be something acceptable. Even contractors work for multiple companies which may have differing levels of Unity version use.. yes/no?