It's hard to understand what exactly you are trying to accomplish. The way I read it, if you have a subsummary report, no body, each line is a group of records, summarized.
So you then want to take the value (total?) of those records, go to the related records, and put the one value into all the records?
I guess you could use Replace Field Contents, but it seems to me you are destroying (changing) all the initial data.
In the subsummary view there are two fields, Category and Assigned to. The user has to change the category and who the tasks is assigned to. Once the assigned user has done what they need to do for that task, they would go back to the subsummary view and assign it to the next person, or change the category if need be. Each task can have any number of records attached to it, and all the records will need to be updated for that task (subsummary group).
It would be simpler to link the entire group of records to a single record in a join table such that assigning the group of records to a different person, changing status, changing category for the group, involves updating that single record rather than looping through individual records to make the same change to each. Whether you loop or use replace field contents, it not only takes more time to do, you might encounter record locking issues if it's possible that other users might be editing the same records at the same time.