Hi there!
I think you have 2 options;
1. Cancel those round-trip tickets and book one way ones instead.
Then get return ticket at FujiQ any time you want.
note: There is possibility that there are no vacancy during the holiday season in Japan.
2.Hold your booked tickets just in case there's no available seats, and take the one way to FujiQ.
And get new return tickets at FujiQ any time you want, then cancel your booked tickets for return in the same time.
They don't take any extra fees for the cancellation.
If you are worried if you will have no seats to coming back, there is so many options around FujiQ area.
You can find some cottages and hotels surrounded by woods, with great view of Mt.Fuji, and of course they almost have Onsen too.
You can try hikes or drive around in the nature after the exciting amusements in the FujiQ.
Hope this will be your help!