DAX: The zhian'tara is a ritual where joined Trills get a chance to actually meet their previous hosts.
KIRA: Meet them? How?
DAX: That's where you come in. If you don't mind, I'd like to borrow your bodies for a few hours.
O'BRIEN: What do you mean borrow our bodies?
DAX: If you agree to participate, the memories of one of my previous hosts will be temporarily removed from the symbiont and imprinted onto you. You essentially become one of my hosts for the duration of the ritual.
BASHIR: And how is this accomplished?
DAX: The memories are transferred telepathically by a Guardian.
After multiple hosts are inserted and removed telepathically by the guardian, this dialogue happens.
CURZON: In that case, you'll be happy to know that I've come to a decision that's going to benefit both of us.
DAX: What do you mean?
CURZON: I've decided to stay where I am, in this body. And I'm not just speaking as Curzon. This is Odo's decision as well. We like what we've become and neither of us wants to go back to the way things were.
GUARDIAN: There's no way to remove Curzon's memories from Odo without his cooperation. He has to give them up willingly.
SISKO: What'll happen to Jadzia if we can't persuade him?
GUARDIAN: It'll take some time for her to adjust to the new balance, but she'll be fine.
SISKO: Do you want me to talk to Curzon? I know him better than anyone. I might be able to get him to cooperate.
DAX: I don't think so. I don't think I want to do anything.
SISKO: What do you mean?
DAX: I mean I'm fine, and obviously Odo and Curzon are well suited to each other. Maybe it's better this way.
So we see that the guardians can transfer memories from hosts to hosts and that process can be temporary or permanent without difficulty. It's difficult to remove a trill from a host and place it in another.
Why not just take the memories out of the host, or out of the trill and place them into a new body as needed? Why bother with all of these symbionts? In the least they could share lots of hosts with more people. For example Jadzia could get a symbiont but the could take the memories of a couple of her hosts and transfer it to others of her species so more could partake at least in some part of the joining process.