thanks to all for their advice... in short: i'm not gonna do it. i (somehow, through my brilliant agent) managed to convince BA that they should "delete" that one segment of my itinerary. the price for the overall ticket remained the same (so BA didn't lose out) and i get to travel by train on the segment i wanted and don't get the remainder of my itinerary cancelled out. not sure how common this is, but i'm delighted that BA was helpful this time...

happy travels, everyone.