I don’t know how many miles you have and how eager you are to spend them. In my experience you often get upgrade rates that are so high that you’d spend less miles on an award ticket in J than an upgrade from Y to J. I personally would not purchase a ticket with the thought of certainly upgrading with miles.
But the best way to find out if DL can upgrade you with FB miles is to give them a call. Upgrading before check-in is normally cheaper than at the airport (with some exceptions). You can also use them for an upgrade when onboard, but my experience is that this is more expensive than at check-in.