You do not need to update all of your machines to version 7.0 of the Project client at the same time. However, machines won't be able to update to the new version of Project client (7.0) without updating their OCR codes because the earlier codes aren't supported there. If a machine must stay at version 6, then they will have to keep their current OCR codes, but upon update to 7 of the project client, they must update. Version 6 and 7 of Project client can interoperate on the same projects, though, so you can choose to upgrade on a machine by machine basis.