I'm fairly sure there's no way to go from Generations Ultimate to Generations since the former has more content and a different save file. It's maybe possible with the Japanese versions, since there's a 3DS version of Double Cross. I feel like the official transfer tool works both ways for Double Cross, but I'm not 100% since I just sent it to the Switch and never looked back.
Edit:
This does seem to suggest it's possible with XX -