As others have said the key question is whether this is the end of your itinerary or only the outbound part of a return. If outbound, I don’t think ba will allow you to cancel just one segment, I believe they’d make you cancel the itinerary and rebook as two one ways, which would mean a need for availability. If you don’t cancel the leg, you’d both lose anything after and risk AUS only checking you in to your (original) final destination.

If you do cancel, personally, I’ll admit I wouldn’t consider either the lcy flight or going via LYS, both of which are quite inconvenient if all you want is a few days in Paris, I’d just buy the paid ticket on either BA or AF and travel hbo or failing that planning ample time between the flights.

Last option (slower but easiest if you can’t cancel easily) go to Spain as planned and then fly from there to Paris.

This thread illlustrates nicely à point I was making recently that award tickets are very flexible in some ways but not at all in some others...