I think the plan, if I were in charge, would be -
Ep7 - Bring back all the OT cast right from the start whilst introducing the new cast
Ep8 - Have the OT cast start to take a step back and allow the new cast space to develop their own quirks so we're getting a 50/50 split
Ep9 - Have the OT cast bow out (dead, old age, happily ever after etc) whilst handing over to the fully developed new cast
However I'm just a fan so why would LF/Disney listen to me
Regarding Ep9 it'll be interesting to see the state of the SW franchise once it solely rests on the shoulders of the new cast without the opportunity to bring the old cast back. I'm hoping the new generation of fans (I'd guess there is some) love SW as much as I loved I did.