Okay so here's some more detail - sorry I should have probably lead more with this...

The ticket was originally purchased on AA stock - outbound on BA in Club and return on BA in WTP. LAX-LHR-LAX. Bought long ago - helluva deal for $1700!! But we're actually going to BCN (LHR-BCN on separate PNR). Now I need to cut my trip short and get back from BCN to LAX a week earlier for work.

Changing the ticket on AA.com shows a fare difference of $2448 - cheapest option in Premium Economy LHR-LAX return leg. But when I look at BA RT fares BCN-LAX-BCN, I can get WTP for E750. SO basically what I'm asking is, will I get into trouble if I throw away my AA return leg (on BA metal) from the original ticket, and throw away the BA return fare from the newly purchased RT ticket from BCN? This will save me a lot of money!

Thanks again everyone...