If you don't want WXYZ to be seen at all, then you do not need to give them drive letters (DrivePool will still be able to pool them together) - Instead of clicking "Change" simply click "Remove" when dealing with the drive letters.
Probably the easiest order to do this if you want to keep every drive with drive letters:
Change F to D Change E, G, H, I to W, X, Y, Z Change J to E (now that E has been freed up)
Probably the easiest order to do this if you want the pooled drives to have no drive letter:
Change F to D
Remove Drive Letters E, G, H, I (see my picture for the Remove Button) Change J to E
You should be able to keep DrivePool running during this whole transition phase (you don't need to remove drives from the pools). Personally I'd go with Option 1. While there are Folder Mounts that you could use, I think it would just be easiest to keep everything easily accessible the "normal" way. Plus without Drive Letters you won't be able to add non-pooled content to the pooled drives (just incase you wanted to do that)