ATF flush vs. drain

[Nobody]

Hi all, I need little help from you. Maybe I did not feel exact difference between flush and drain. My tranny was rebuilt couple months ago but was filled up with some Valvoline semi-synthetic ATF insted of Mobile 1 I wanted and insisted (you know some mechanics are silly :-((() It has some 2000 kms on it but I will want definitely switch to fully synthetic. So question is what is RIGHT procedure for me? Just to let drain it out and fill it with synthetic? Or what exactly mean FLUSH. Thanx for help.
Peter