My children both have autism, diagnosed when they were toddlers, and as our youngest grew (he's now 14) there were things about him that seemed not his autism, iyswim
. It's really hard to describe because autism and adhd share quite a bit and there is an overlap and it isn't until they are older that it becomes clearer. Mainly his activity and energy levels, his attention span, and just our own understanding and experience. So we raised it with his paed and eventually he prescribed equasym xl.
The difference is noticeable! not since they came off gluten have we seen such a big difference!
He focuses and interacts MUCH more and he is generally calmer.
The promised appetite decrease has not happened yet though 
So now he has a dual diagnosis.
I would say one thing - there is nothing to be gained by waiting to assess and much to be lost.
An assessment takes time. If you wait until you are in crisis/the child is not coping/things are very bad then you have lost valuable time.
An assessment does not hurt anything. It doesn't give them a condition they don't have and if it turns out there is nothing - your mind is at ease.
However, if an assessment shows there is something, then you begin the work that is needed in order to help the child. And the sooner this is done, the better the outcome for the child.