Thank you all. So, I have a discounted economy booking man-lhr-las return.

The las-lhr leg just got availability for a business seat. I can't upgrade due to the fare class of my economy ticket. So, I can grab the business seat, but when I get to lhr the final leg will be cancelled.

I'm assuming that there's no way to "hold" the final leg given that I will still be on the same flight, albeit on a different ticket.