@AjitKumar is correct. You cannot currently step the upgrades. Is there any particular reason why you'd want to do this? If it is for stability testing do you have any other networks with fewer APs? We typically do firmware updates on those smaller networks first to make sure things are stable before rolling it out to our sites with more APs. One potential, but not ideal, option would be to split your devices into two networks. But that would be only if you absolutely have to do them separate and have no other networks.
Adam R MS | CISSP, CISM, VCP, MCITP, CCNP, ITILv3, CMNO
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