"formerly unconditioned interstitial space between 1st and 2nd floor into a conditioned space"

It is already conditioned space.

Anything within the buildings thermal envelope is conditioned space, even if you do not directly heat or cool it.

As brickeyee said, you are already heating that space. The R-value of the flooring above is maybe R-2 to R-3, and the sheetrock below is R-0.5. If it isn't well sealed from the outside, adding holes in the ceiling will create some additional loss, but it is already about the same temperature as the house -- maybe warmer if you have heater ducts in there.

Great, thanks guys. My HVAC ducts are in there - insulated flexiducts.

I went and got the non-air tight trims (Halo 353SN - very sharp)