You can do it in this way:
- Put the rideable Koraidon in your Party and Save.
- Open your Save in PkHex, go to Party and edit Koraidon to Miraidon, Changing only:
Species (Koraidon to Miraidon obv.),
Met Origin Game (Scarlet to Violet),
Moves to match Miraidon ones,
and finally (if you have) change the OT Name SID and ID to match the current Save.
The last one is only if you changed your ID to a new One during conversion (I did it so that the saves are different).