for me its happened a few times, aisle for aisle no problem, anything else might be a problem, but doing a seat change on BA out of BOM to LHR, worked out very well for me,
the FA asked me to accomodate a passenger, did it, and then another FA put asked me to reaccomodate another passenger, so basically lost my bulkhead seat with lots of legroom,
the first FA comes back, sees the sistuation, tells me, he will find me a better sit, right after take-off.. he does, puts me in business...