Yes, warranty could be an issue with the mismatch. Payne is pretty similar to the base model Carriers. The ones I've seen appear to be better from what they were in the past, so not bad units, but there is better (construction wise). If cost is an issue and limiting you from putting in a better matching unit, then I would look at Payne or Goodman. That way, in time, you'll be able to replace everything with a matching setup.
Still, how do I find out if the attic unit has a TXV?
Well, one of two ways. Go up and look yourself, or ask your tech.
I'll bet the next question is, what does it look like?
Maybe someone has a picture they could post, or maybe go to the Sporlan, or whoever these days, web site. I know exactly what it looks like but, would have a hard time explaining.
I think Ryan said it's an obsolete coil now so, I don't know if he could even pull up a picture.