My concern was that when I tried to touch the 9.0.x serve with the 8.5.2 admin client I'd get some kind of 'nope...not allowed to use that old thing on this shiny new server' alert.  I really don't do much on the portal side anyway; it just sits and runs so worst case I might have to issue commands on the portal from the server console.

Thanks.

...at least for a Domino 9.0.1 server...

Actually, come to think of it, my Administrator Client PC IS still at 8.5.3FP2.

The only thing that I can think of where you might run into an issue is, if you were to set 'Create_R9_Databases=1' on the server, you would not be able to copy any database that had been converted to ODS 52 on your R8.5.x client and open it. If you did not set 'Create_R9_Databases=1' (for example, still used 'Create_R85_Databases=1') you should be in great shape.

Since you are upgrading your server to 9.0.1, you should upgrade your Administration Client to 9.0.1 as well.  Both the 9.01 server and Admin Client will be backward compatible with the 8.5.3 servers.  You can sometimes see strange admin "Quirks" when trying to administer a 9.0 server with an 8.5.3 Admin Client.