Sold domain and website transfer process

I recently sold my website that was being hosted on dreamhost. The buyer of the site insists on transferring everything to his host, which is Media Temple. I was just wondering, if we transfer the domain name, will that disrupt the site? Or does it only disrupt the site when we change the host? How long will the site have to be down before I can upload it to the new host? Any guidance on the best way to do this would be much appreciated. Thank you very much.

First you upload a copy of the site at the other host and make sure everything is working properly. Hopefully media temple has the use of mirrors so you can view your site before you change the dns settings of the url so you can check that everything is working. Once you are sure everything is fine THEN change the dns settings. So long as you wait before removing the files on dh for 24-48hours there should be little, if any, noticeable downtime while the new dns settings are propagating.

I hope you got paid first btw.