Well, I got support to reset my licenses and was able to activate them on the License Manager. Unfortunately, I learned that it is not possible to use the same license for either version. Once upgraded the license can only be used on Studio 2017. This is the response from SDL Support:

The license for Studio 2017 do not work for Studio 2015 and the upgrade process replaces 2015 license with 2017.

Hi Octavio,

There seems to be some confusion here... most likely on my part?  My understanding from support is that you wanted to upgrade Studio 2015 licenses to 2017 and then for the Studio 2015 client machines to borrow seats from the 2017 network license.  This is not possible.  You can use the License Server Manager to license several products at once, but you need a separate license for each product.  If you only have a Studio 2017 network license enabled, you can only connect Studio 2017 to it, and the same goes for Studio 2015.

I think the part of your question where you said "Can the 2017 license server support 2015 and 2017 simultaneously as our users upgrade?" is the key part I didn't get the first time.

Is this your understanding too?

Regards

Paul