Its totally up to you. I know in my headcannon, Fire Convoy is a different character, so if you want to make Nightwatch Prime a different character, go for it!
I just like the idea of Optimus hunting down someone with Movieverse Prime's violent, face ripping tendencies.
In my headcannon, Optimus prime, G2 Optimus prime, Laser Prime, and Hero Prime are all seperate characters, so I see no reason nightwatch Optimus can't be his own thing.
I mean, G1 Optimus shares a body design with Ultra Magnus, so there's certainly precedent, right? Somebody just needs to come up with some faux Latin superlatives...maybe he's Optimus Tertius...Noctus Ultimo. or some such...
Their your toys, you can call them whatever you want. For instance in my collection Kingdom Galvatron is Galvatron and Selects Galvatron is Ted from Accounting.