You need to be a little more careful going down in caliber than you do in going up but the key is to do it in stages so you don't buckle the cases and for some of the more esoteric modifications it is better to get "form" dies.

One other aid is to anneal the necks before you begin.

There are folks expanding 300WSM brass to 416 with no apparent problems so, no, there probably isn't a practical limit.

If necking up you need to keep in mind that the shoulder of the smaller calibre will be forming part of the neck of the bigger calibre.

If extreme accuracy is the goal then I think necking down by even one calibre, say 30 to 270 or 338 to 30, is not a help to accuracy unless some stress relief is done. That could be either by annealing or a light neck turn.

As to the mechanics of necking up and down....calibres like the 30/06 can stand a lot more than calibres such as the various Weatherbys. The Weatherby shoulders will collapse if too much force is applied to the neck whereas the shoulder of the 30/06 will resist a lot of force. I would imagine that a 300 H&H could be necked to 17 in one pass through a die without the shoulder collapsing

Mike

I have Wilson vice dies for 7mm Mauser and 257 Roberts.
I can size 8mm down to 7mm and then down to 257 without annealing.
Then I can fireform 257 Roberts Ackley Improved 40 degree shoulder, with still no annealing, and great accuracy and case life as good as the 257 Roberts +P brass.

Going down in the neck area is not that big an issue until you go a couple steps. Using a case were the new neck is in the body area of the old is often an issue. You might need to turn the necks. As to going up. I take a 280 to 375 all the time. I do it in 3 steps with tapered expanders. Only failure is if I don't lube well and the shoulder collapes. I see no shorter case life. I have also gone up to 416. Lube well and go slow.