I have a BA-issued DONE3. I'm considering changing one of the segments from a BA code (as ticketed) to the JL code share when D class for the latter becomes available. My question isn't about use of code shares - my itinerary features several of them, though I acknowledge others have reported difficulty booking these. My question is about the consequent change (in my favor) for YR/YQ charges - JL's are about half those of BA. At re-issue will that mean I receive a small refund?

There should be no Explorer change fee, as I'm not changing the ticketed points, though I half expect BA to impose a service fee for the change.