Following bc im in the same situation
Agree with @Ellie-Jo. Start with the naps and spend some time in their room playing, reading, so they get used to it. Also get good blackout curtains, those are a must in my opinion.
We did this transition and we would just take turns on who would go in each night and help him go back to sleep or sleep with him if he was to restless until he got more used to it. We got a queen bed and put it on the floor so we could still lie down with him when we put him to sleep and when he needs it. Took about a week
We also introduced a couple lovies/teddy bears to help supplement missing cuddles
Following
If you don't already, try day naps as you'd have night sleep