Can you upgrade it to 9.1?
You should be able to publish it directly in 9.1, at least in a .NET environment. Did you try?
Yes, .NET works fine but the java doesn't update automatically.
Java stack does not upgrade automatically.
So you need use Service Studio / Integration Studio to open the eSpace /Extension.
It will prompt you to upgrade the module, upgrade it and then you can publish.
I've done it already, but i still think, if possible, you should create a version for 9.1.