With any sizable number of instances to upgrade, where most are undergoing version upgrades, it may not actually make sense to use the same servers. Servers acquired in the timeframe of "relative newness" of some of the older versions of SQL Server, may not be adequate to the needs of more recent versions, and in addition, you run a significant risk that if you run into a problem that can't be fixed, going back to your old setup could be extemely painful if not impossible, without an awful lot of precautionary measures that would significantly elongate the process. Also, with version upgrades, it's generally a good idea to run in parallel for some period of time in order to ensure that everything still works in the new environment. Personally, I wouldn't even consider it.
That said, I wouldn't necessarily object to re-use of some of the hardware, assuming that it's of sufficient capacity or can be upgraded to that level, but only after the old system has been sunset after a parallel operation sufficient to justify the sunset. Thus one could do a sort of "rolling upgrade" with at least some of that hardware.