Please check
I would urge you to check as I had an experience with FR some time ago - must be 6 years now, when my outbound flight to Treviso was cancelled. I made alternative plans to get to Italy.
When I presented myself at Treviso for the return, they wanted to charge me again as I "had not taken the flight out" When I pointed out they had actually cancelled it, it cut little ice with them. It was only after a lot of phone calls and arm waving that they allowed me to travel "just this once" without what they described as a "penalty"
As with all things FR, I think it best to check!! But to be fair in those days I use to use them 8/10 times a month and they rarely let me down