Sorry doesn’t work that way. To declassify a document, the document itself needs to be remarked. If Trump is in possession of a properly remarked (declassified) document, Biden can’t just waive his hand and change that classification.
THE DOCUMENT ITSELF needs remarked. Now if Crappy Pants’ FBI Goons remarked the documents after the raid that’s Illegal.
Bingo! And easily proven.
(Unless somebody has new technology)