Hello
my dd1 (ASD) was 3.4 when we moved her out of her cotbed into a proper bed. She is very tall for her age, but still just about fitted into her cotbed, and seemed comfortable enough.
I had dd2 by this stage, and when dd2 was 4/5 months old and needed a cot (dd1 was 2.11 at this point) I did wonder about moving her, so that dd2 could have the cot
I decided against it, because I wasn't at that point ready to deal with the resulting problems, should they occur. So we bought dd2 a cheaper cot. we did get a good mattress for it though, as we would have wanted to get her a new one for the cotbed anyway - after 3 years of dd1 jumping up and down on it, we thought it had done it's time
I ebayed dd2's cot and dd1's mattress (are you still with me? ) once I did move the girls around, and got back two thirds of the cost of the cot, which I was happy with.
FWIW, we had a few nights of dd1 being a bit unsettled once we moved her to a big bed, but we soon left her to it - she used to sing to herself, and look out of the window etc until she fell asleep, and then I would go back in on my way up to bed and put her in bed (she fell asleep on some cushions on the floor for about a week or so) and tuck in etc. After about 3 weeks, she was falling asleep on the bed 95% of the time, and she is as happy as anyhitng.
It made going away/staying with friends a lot easier too, as she used to hate the travel cot, but sleeps happily in any bed, as long as we take her blanket along
gosh, I've gone on a bit
hope it has helped a little