Are the Extension Attributes statically entered, like a text field that is filled in during a Recon or at enrollment or something? If so, it should be possible to update them all through the API, but it would need to be scripted and would still be a lengthy process (though more automated)
If they are Extension Attributes that get populated via a script, although still possible to update these, again using the API, of course the value could and would likely get overwritten when the device next submits inventory. So I'm not sure how reliable that will be in the long run.
Also, by "devices", are we talking about iOS devices or Macs? Or both?
Taking a step back, it might be better to look at another approach here for moving devices into and out of groups than using EA values. If you need to do something like this regularly, I would explore some other workflows perhaps.
