(Oops, maybe I just started something here!)
My 15mo (Aaliyah) was litter trained. I had no problem transitioning her to outdoors. I initially had 2 litter boxes set up - 1 upstairs (in an xpen with her crate) and 1 downstairs. Almost immediately she preferred to go outside - only ever pooped in the house (in the litter box) once. So I removed the one downstairs and kept the one upstairs until she now longer used it (around 6 mo). There was no confusion at all about going outside - when she was out, she went - especially if on a walk - because of the excitement.
I'm in Maine - so it was cold and snowy when she first came home - really, really snowy last winter. She loved being outside and was always the last one to want to come in. I actually credit her not having any care about weather (snow or rain) because she started playing outside at 12 weeks. To this day even if it is pouring she doesn't put up a fuss about going out and is actually excited - unlike my other 2 who go out begrudgingly.
Oh, and I used Equine Pine pellets for the litter box. Same as Feline Pine, but can the Equine version can be purchased in 30lb bags at Tractor Supply for about $6 a bag.
Maggie is doing much better about going outside. She's using her litter box less and less. We had a ton of rain last night into this morning however, which caused her to use her little box this morning. By the afternoon we had her back outside. She's basically refusing to go out in the rain and stands in the yard with one leg up if it is even a little bit moist. We are using corn cob bedding as she was the only one in the litter who had a reaction to the pine bedding. I guess I'll keep the litter box around for rainy days. 🙂
