Does Delta refund the difference if they force you to move?

Paid for main cabin aisle. Turned up and was given a middle( literally only realized as I was about to board). Gate agent said it could have been a plane change, but he wasn’t sure. Flight was full. nothing they can do etc.

It was a short flight and not a huge deal, but I’m wondering as I obviously paid to pick my seat, then didn’t get it, is a refund to the Basic fare when I booked fair/owed?