Have you tried an activity that opens your work object, iterates through PageName.pyAttachments() and if the .pyCategory and .pyLabel values == "File" change them to "DealInfoDocument" and then save with either write now checked, or a subsequent commit to get the changes to the database? That won't update the Link-Attach object, so you may need to do the extra step in the iterator of opening that object and updating properties there too and then saving it to the database.
I'm not really sure what more to add. You have a list/group PageName.pyAttachments(), create an activity with an iterator that iterates through all instances of that list/group. From within the iterator check the values of .pyCategory and .pyLabel. If they equal the string "File", change them. Then save the change to the database. If you're running into trouble, a screen shot of what you have and what is going wrong might help.