I dont think it matters if it's a spare or not. I mean, you can always say the spare become primary just now, because you gave away or lost the other one, etc.
However, in your position I would not give up on that one size smaller fit just yet. One size difference in those solo loop brands is really small, and over time they will actually become a bit looser too. My recommendation would be to start using it anyway, maybe just a couple hours every day. During swimming workout could be ideal perhaps. In my experience sleeping requires the loosest band (or no watch at all), but especially with cooler days and swimming a tighter band is a better fit :)