This would actually be a big change as a lot of underlying code is effected. Damage would have to count down, healing would have to count up, colour coding for damage would have to be changed, etc., etc..
Keep smacking your players around the head until they get it. ;-)
EDIT: But, you may also want to add this to the FG wishlist so the devs can at least consider the option for future inclusion. https://fg2app.idea.informer.com/

