I have 100 once fired Remington 30-06 cases that I am considering necking down to 25-06. Is this easily accomplished or something I'll have to do in more than one step? If I have to turn necks or anything of that sort it will be easier for me to just buy new 25-06 brass.

It should be easy enough. Pull the decapping pin out of a full length resizing die in 25/06, lube and run it in. Check for length, trim if necessary and your done. A friend of mine does exactly that although recently he's been using 270Win cases as they're currently cheaper than 30/06 and much cheaper than 25/06 cases.
Cheers...
Con

You can do this but it is more trouble than it it's worth unless you have nothing better to do.
I used US military match brass for this and it worked ok. The necks were no thicher than Remington commercial brass. I would lurk on eBay to see what kind of deal you can get there.
Actually I would use .270 brass since it is much cheaper once fired. The .270 brass is longer and can be trimmed to match you chamber length if you know how to measure it.

Just ignore the fact that it's .30-06 brass and run it through your FL die (properly adjusted, of course, for the headspace requirments of your rifle). Don't know why Con advises removing the decapping pin? You'll need to deprime, just as with any once-fired brass, and you'll also need the expander ball to pass back through the necked-down brass to create the proper inside dimensions.

Lube properly, but don't overlube as you'll get wrinkles in the shoulder.

Necking down '06 brass has three advantages for the .25-06 shooter: (1) You can set the shoulder for a perfect headspace fit in your chamber by running them into your sizing die only far enough to allow them to snugly chamber, (2) you can typically get a slightly thicker neck that USUALLY fits your chamber better (but always check to make sure that the necks are not too thick), and (3) '06 brass is usually cheap and plentiful.

Some people worry about the headstamp. If you can't tell the difference in a .25-caliber bullet and a .30-caliber bullet, then you probably can't see well enough to read the headstamp Besides, an '06 won't even start to chamber in a .25-06, and a .25-06 is not particularly hazardous (at least to the shooter) if fired in a .30-06 chamber.

Been doing this since the .25-06 was a wildcat. Have found that if I use .270 brass, the necks don't need reaming or turning, the shoulder can be better set, and as was said before, I can trim to a perfect length.

Stonecreek,
Purely a brain fart... your right, decapping pin doesn't need removal when necking down.
Cheers...
Con