I've sized 30-30 to 219 Wasp and 7 Waters, and 32-20 to 310Cadet, and I'm working on a project that will use 22Hornet shortened and necked to 25 cal.
its really no big deal sizing up or down you just have to keep in mind that someone might pickup your cases, hopefully they are smart enough to look carefully.
I've seen 30-30 fired in 303... not a good thing for accuracy.
But for 270, I have a bunch of brass and I don't own a 270, I also have a bunch of 30-06 brass and don't own anything in 30-06. If I was desperate for 25-06 brass I would not hesitate to take the 270 and neck it down.
Same issue for my 325WSM, brass is hard to find, I would not hesitate to take 300WSM and neck it up, but I bought 400 325WSM cases so that should do me for life.
one thing to watch when necking down is the thickness of the brass, you might need to turn the necks to thin it, not an issue when necking brass up.