Just a couple of quick stories i'd like to share......
One student at the school i attend was given a higher belt when he joined - mainly because he had done a very similar form of training previously. I also believe he was the first one in our school to have this done.
It back fired though - it took him an entire year to progress through to the next rank when it should have only taken 3 months. The reason was simple - he had to learn all the basics again in our style, and surprisingly it takes a year for a beginner to get to where he started.
So it was a first and a last...........
On a similar topic i was in a sparring class the other day and i was standing with a white belt. It was obvious that he had done some training before because of the way he threw his kicks but when i enquired he denied he'd done anything.......so i trusted him. Then we started to incorperate takedowns and grappling. So because i was under the impression he had done nothing before i told him how to get in a takedown using what a white belt in our style would know - and then allowed him to get it in. Basically i wanted to allow him to have some fun with the grappling even though he hadn't gotten to that part of the corriculum.
What happened next was disturbing - he did the takedown, got into side control and put on a shoulder lock........and distroyed my shoulder.
Naturally i cracked it - not becuase he submitted me (i'm not the greatest grappler ever so i get use to it) but because he told me he knew "nothing" yet put on a shoulder lock............
So in my opinion - you start a new style then approach it totally humble and wear the white belt with pride.
However be totally honest with everyone about your previous experience - as it is something to be proud of and allows them to guage what they can and can't do with you.
But again, just my opinion.....