bit to hackamore

["The Vervaet's" <VervaetP@deruyter.k12.ny.us>]

VervaetP@deruyter.K12.NY.US
I want to try a hackamore on horses that have been previously ridden in
bits (mild types). Any suggestions in change over?  Do I just "do it", or
is there something that I should do in-between to help adjust them to it? 
What type would everyone recommend....mechanical hacks.....or other types?
Thanks
Paula