From about 13-15 months we just started doing day sleeps in their own big bed. And somewhere in there we started putting them initially in their own bed at night too, but then they'd bring themselves in to us during the night and co-sleep from there. But each sleep started in their bed with the explanation that they could call out to mummy or come in to mummy if they woke up. They eventually stay there all night, and then lots of praise is sung for staying in their own bed aaaallll night My DD still comes in during the night, with the frequency of sleeping right through slowly increasing. They've always known that they're welcome to join us tho if they wake.

They're both old enough to be told now tho that they can come in for a cuddle and mummy will take them back to their bed, and they seem ok with that most of the time (tho laziness on my part usually means I just tell them to climb in to the middle hahah)

We haven't had any real dramas with transitions to their own bed. I think as long as they still feel welcome to join you then they can enjoy the novelty of it without feeling kicked out, if that makes sense.