icloud storage

I am already in a Family icloud storage scheme of 200 gb . This storage is almost. full.

so i could. actually increase my storage space by paying. a little more ,but my son already has a bigger storage plan with apple of 2TB and he does not use all and he has asked me to join his Family sharing i cloud storage plan. So when he sent me the invitation and i clicked to join _it is telling me

"you are already in another. family" - you can be in only one family at a time. To join this family , you need to leave your current family first"

so my question is how do i do it without losing my storage of the first family

thanks mushtakali