[samibe]

It's easy enough to filter out songs and use batch edit to fill out the data for empty fields. It's also easy to add more entries to a field to add more granularity to the search options for a field. For example, I have hundreds of duets (that I have labeled as "Duet" in the "Instrument" custom group) that I could then go through and add 1P4H (one piano four hands), 2P4H, PianoOrgan, GuitarPiano, etc. by further filtering the songs with a "Duet" instrument and doing a batch edit. It doesn't work as easily the other way. 

Scenario, say I have a Collection of all of my duets (of various instrumentations) but they are listed in the Instrument field as 1P4H, 2P4H, etc. only and I decide I don't want a Collection of duets, I want to add "Duet" to the Instrument field. 
 - Option 1: Go song by song through the Collection adding "Duet" to the instrument field and clicking "Save and edit next"
 - Option 2: Filter the collection down to a set of songs that have the exact same instrumentation, select them all, batch edit them, make sure the field name isn't red, add "Duet" to the field, click save, and repeat for the other instrumentations in the collection. 
 - Option 3: (Slightly faster than option 2) Go the Instrument tab, select each duet configuration (1P4H, 2P4H, etc.) one at a time, select all the songs in the list, batch edit them, make sure the field name isn't red, add "Duet" to the field, click save, and repeat for the other instrumentations. If the filed label is red and the data can't be replaced for all the songs in the list, then I have to close out of the batch editor, filter the songs further, and try again. (Which sounds a lot more like option 2).

It would be easiest if I could select all the songs in the collection, check a box at the instrument field to add a value to the field without changing any existing values, type "Duet" into the instrument field box, and click OK. 

Alternatively, rather than having the field label show up red when the values aren't all the same or have to check a box to add or replace the values in the field, I think it would be more helpful if the data that isn't shared by every song in the batch were to be grayed out. Checking the box (replace) that is already there would make all the values active (not grayed out). Selecting a grayed out value would make that value active (not grayed out) and add that value to the all the songs in the batch it's missing from. Adding a value to the field would add that value to the whole batch and not change any other value in the field. In terms of the scenario, I could select all of the songs in the Collection, edit the batch, and see that there are a bunch of grayed out Instrument values. I could then add a "Duet" value to the field or (if it's already been added to a few songs) select the grayed out "Duet" value that is already in the field so that it gets added to all of the songs in the Collection.