I've had a pretty easy time transitioning from one-to-one to one-to-many by just keeping the old 1-1 mapping, renaming it as the "primary" option, and then addi

I've had a pretty easy time transitioning from one-to-one to one-to-many by just keeping the old 1-1 mapping, renaming it as the "primary" option, and then adding a list of secondary options. All of the old functionality continues to work, and any new stuff that needs multiples can operate on "[primary] + secondaries".