I haven’t had the experience described above, but that doesn’t mean this is the rule, LHG rev mgmt / IT is weird and I wouldn’t be surprised if this happened.

I almost exclusively book full flex tickets nowadays, and never had a problem making (numerous) rebookings.

If you’re already in booking class A both inbound / outbound, could be new (cheaper) fares were loaded in the system in the meantime.

No need to overthink it: call to ask the price diff for the rebooking, if you’re not getting any money back, make a new booking and scratch the old one.