We were also unsure of when it would be best to do this for our DS1 -26 months old when DS2 was born. I went backwards and forwards about doing it before #2 arrived but in the end i decided to keep him in his cot (i was able to borrow one).

A short time after DS2 had arrived we started leaving the cot side down for DS1, expecting him to come and go as he pleased but it took a while still for him to even get in or out himself. Eventually after us showing him a number of times that he was able to get in and out himself he started to do so.

There have been a few occasions (i can't remember if it was when he was in the cot or his bed) when he has gotten out of bed and hopped in our bed (before we were in it) or decided to start playing with his toys again. It has been very rare though. We don't have issues with him continuing to hope out once we put him in bed.

DS1 would also move around in his cot a lot which made me wonder how he'd go in a bed but he has been fine. He is in a King Single and we don't use a bed rail. He's fallen out once and when we ran to check on him he was curled up on the ground alseep. It didn't wake him

There was a time when he'd wake up and come and hop in our bed, it was a nightly thing at one stage but i never made an issue of it, let him have a few minutes hug then took him back to bed. I didn't want to make him feel like he couldn't come to us so i always allowed him to have a quick hug. He doesn't really stir that much anymore but if he calls me i go.

So to wrap it up DS1 was in a 'big boys bed' at about 2.5yrs and it was a really smooth transition. Much easier than i thought it would be. Hope you have a similar experience.