Hi, when you have an ecoinvent license on Nexus, you will always have access to the previous versions. They are not necessarily fully "compatible" since ecoinvent changes and updates things, and we do not update all older databases to the latest openLCA version, but you will be able to access them.
You do not need to purchase the non-Nexus license since you already own it, you only need to register this same license in Nexus, which will make it a Nexus license. This license is then for a given ecoinvent version (e.g. 3.9). When you want to get access to newer versions, you do not need to buy a new license but only need to extend the maintenance for this license in Nexus.
I am assuming that you are not talking about the yearly license which simply expires after one year.
Best wishes,
Andreas