Hello and sorry to take over your thread

I see this thread is about the 9650.
I searched the Forums for this but no one seems to talk about Reloads with the new DM for Mac (December). Since previous versions of DM for Mac are clunky, i thought this new DM should work differently..
I finally got my first BB Bold 9650 and i have a Mac
Well, i installed DM on the Mac and now i'm faced with a dilemma.
The phone came with OS 5.0.0.975 and when i plug in my phone to the Mac, it gives me the option to Upgrade to OS 6, to RELOAD OS 5.0.0 or to Downgrade to 3 other OS 5 versions.
I can see a lot of upgrades that need to be done, both in OS 5 and OS 6.
What does the RELOAD option do exactly? Does is wipe the OS and install a new one, removing BBM contacts and stuff like that?
Or is it a smooth/quick operation, and everything remains the same?