Mixing conventional and synthetic ATF

It's probably been discussed and after using the search feature I couldn't find (or missed) the answer so here it goes,

I'd like to switch over to synthetic ATF in my 02 but there's conventional ATF in there now. Would a series of drain/refills be ok to eventually get all synthetic ATF in there or do I NEED to do a flush to get the old conventional stuff out before I go to synthetic?

 

Did this to a Chevy Silverado

http://www.youtube.com/watch?v=MsdPAadc9fY

I didn't have access to a compressor, so I didn't do that part.

 

Drain and fill and you will be fine.