mass-upgrade admin feature for plans

Not sure what more to advise as the import tool isn't designed to be a mass-upgrade tool. It's possible you may need to change your plans to non-exclusive so you can do the import then change back once done.

OK I see how to do it this way, but how do you mass remove then ?
Let's say that I import Negopme plan subscribers to dummy plan then how can I suppress them from Negopme plan to import them again into Negopme ? or do you mean that reimporting in the same plan will be enough ?
I don't see it really

It's no substitute for a mass-upgrade and there is no mass-remove feature either. You'll have to adjust plans or test different scenarios (do test imports so no change occurs) until you've something that meets your needs. It's probably just easier to individually edit each user and be done with it.