I am only speculating here so please investigate this further but if you are on subscription can't you just (re)download SW2015 from the Solidworks website and install?
Speak to your VAR as soon as the weekend finishes............
The subscriptions of core SOLIDWORKS and Flow Simulation are dealt with seperately, so it sounds like you only renewed your SWX not Flow Sim.
As Dave says there should be no issue installing 2015 and using your flow simulation license on that.
Contact your VAR and install Flow Sim for 2017.
Thanks for all the responses.
Yesterday I figured out that all I needed to do was activate the Flow Express in my 2017 version (which was mentioned in one of the replies).
All is well now.