When the zone category is changed for more zones at the same time then in some cases not just the category, but other parameters of the zones will change as well.
The zone category change always evokes the refreshment of several zone parameters. Those default parameters, which come from the library part will be updated with the values coming from the multiple selection. So the modification of only one thing, the zone category could result in more, in most of the cases unwanted zone parameter changes.
The parameters, which could change in such cases are all Finish parameters (Floor, Wall and Ceiling Finish) and the Reduce parameter of the zone area.
1 - Before changing the zone category for all zones at the same time:
2 - After changing the zone category for all zones at the same time (unwanted changes in the red-marked parameters):
The workaround to avoid these unwanted changes is either to modify the zones one by one or select the zones, which have the same parameter settings and do the change for these selections in one step.
Good result when changing the zone category one by one (only the category changes, the other parameters remain as they were):
NOTE: the issue is expected to be fixed in a future ARCHICAD version.