The conversion of the opposing pieces is automatic - no die rolls, no surrender of pawns or other material, etc., subject to these limitations. Pieces adjacent to the defending King will not convert. Pieces will only convert if the attacking piece or the piece just captured is of an equal or higher rank. Conversion is optional. If the capturing player's game is somehow improved by leaving the opposing pieces opposing, so be it.
Conversions are a by-product of the player's move and capture.
There are no additional moves allowed, the converted pieces do not start moving, etc. Captures en passant count. The squares available for conversion are those around the capturing piece. A player in check may make a capture that, in itself, does not eliminate check, if the capture leads to a conversion that does eliminate check. Converted pieces may immediately give check or mate, if they are so positioned.