Any experience changing the "lead" on a reservation? Tips requested

[VeronicaZS]

My in-laws want to join us on our upcoming trip but they are blocked from making a new reservation at CBR, they have 6 day tickets. During all this craziness I had 2 rooms booked for different dates. I want to transfer my August reservation to my in-laws and change to dates to match my other reservation in October. I am the lead on both reservations. I was told this needs to be done (if it can be done) by guest services and I have not been able to get through on the phone. The online modification is not working for me.
Does anyone have any tips for accomplishing this? The only urgency is they can't make park reservations and I don't want to them to be blocked out particularly if MK reaches capacity on Halloween.