I do not know the nationality of the pax. But many countries keep the maiden name in addition to the married name.
If it is a UK passport and the maiden has fully disappeared from the passport, then you are in a bind. I would assume that you would lose the upgrade. But the process to reinstate the "old" Y booking (issued on 125 stock) will not be straightforward as your ticket is now 160. And you will have to contact BA to make the name change on that Y BA booking as explained in the BA link above.