I'm doing basically the same thing.

HCL Notes/Domino 8.5 Forum (includes Notes Traveler)

Subject:

I'm going thru pretty much the same process. Upgrading clients from 6.5.5 to 8.5.2, also replacing some old servers. The current servers are a t 8.5.x, but really no matter.

You should always upgrade your servers first, the clients should be at or below the server version.

I would recommend:

-Install your new 8.5.2 server(s) first.

Once the server(s) are installed & tested, you can use the adminp process to move users' mail files to the new servers.

Once a user is on the 8.5.2 server, the client can be upgraded.

Feedback response number WEBB8GXH96 created by ~Sigmund Dworelitnivu on 05/17/2011