Please adivse.

I am on an AONE4 and scheduled to fly WAW-LHR on BA (29 August), which does not offer "A". The agent saw that "D" was showing "D0", so booked me on a later flight the same day, with "D" availability.

Am I allowed to change back to the earlier flight and book into "L"? (It showed L availability.)
Is there any penalty for this downgrade?

If waitlisting is allowed, I would waitlist for "D" on that earlier flight.

I am not changing the routing or the sequency, so I do not expect any change-related fees. Just not sure if I am allowed to downgrade, and without fees/charges.

I want the earlier flight so that I can maximize my time at LHR prior to my "A" flight late that night to DXB. (I can survive "L" on such a short flight--WAW-LHR.)