This may have been answered in this thread (quickly read the last 5 pages but didn't see anything on this and also didn't see in the wiki which I thoroughly read) so my apologies as I'm pressed for time.

If I buy a First Class ticket in D class for tomorrow late afternoon but then I see that I can leave earlier can I do confirmed same day change to an earlier flight that currently only has seats in J class available (and is obviously a lot more expensive)?

Thanks!