Changing sourse of reporting group

At my update attendance popup
I have a repeating group that shows each student in the lesson with it’s attendance status
I have a button that should set an “absent” status for each student that have empty status yet

but I need to do the change at the popup without updating the DB ( becase runing backend update is very slow) !

to solve the issue , I thought to change the “data sourse” of the repeating group , by retrieving the exact attendance records from the database ( using “do search for” ) and make the needed change to each of the returned records that has empty “status” , and this should reload the data at RG very quickly with the updated status.

any Idea how can I perform conditional change to a field in the reterned data( returned by “do search for”)