First, thank you for your replies (excuse my english, it's not my language
).
we'ra actually in version 4.5 (yes !) and migrating in 6.0 but I knew (I think so) how to do that and I didn't want to merge problems.
I think there is no really difference between first migrating from 4.5 to 6.0 and after separate master and media or separate and then migrate...
But the first solution seems to bee easier beacause there is one server to migrate not tow...isn't-it ?