I'm replying from my phone so can't go into detail but fwiw I'd be inclined to share the information about your ds' previous diagnoses and also dhare that you have doubts about it (if you do). I see this as an opportunity to get "data" from another professional, not just IQ data but also her impressions of your ds' behavior etc. I think you will have a better chance of getting all-around feedback and valuable information if you are upfront about everything.
Fwiw - we've been in a similar situation with ms ds and an incorrect ADHD diagnosis when he was in early elementary, and sharing the info is how we've chosen to approach the situation, and it's worked out well for us. A reputable tester is not going to purposely change out or skew tests based on a previous diagnosis; they will draw their own conclusions through looking at the whole child.
Best wishes,
polarbear
