Hi, Jeeper!
The .280 Remington/7mm Remington is basically the .30-06 case necked to take a 0.284" (7mm) bullet. It is the ballistic twin of the .270 Winchester, but does not have it's established 70-something years of popularity.
Yes, you CAN size .30-06 cases to make .280 Remington cases, BUT with a condition:
The .280 case is intentionally longer in the head-to-shoulder ("headspace") dimension than the .30-06 for safety reasons so it can't be chambered in a .270 W by mistake.
You need to do one of two things to make safe, useable once-fired brass from .30-06 cases:
1. Expand the .30-06 case neck to the next larger size (8mm or .35 caliber) using an expander plug. Full-length size to maximum .280 dimensions by carefully adjusting the sizing die in your press until the formed cases will barely chamber in your rifle. Trim to length, deburr, and load.
2. Full-length resize .30-06 cases to .280 maximum dimensions by carefully adjusting the sizing die in your press until the formed cases will barely chamber in your rifle. Trim to length, deburr, and load.
USE STARTING LOADS ONLY (always refer to a standard loading manual for primer, bullet, and powder combinations). Seat your bullet LONG so that when chambered, the bullet presses right up to the leede in the chamber throat.
MAKE SURE THESE ARE RECOMMENDED MINIMUM STARTING LOADS. SEATING THE BULLET OUT WILL INCREASE CHAMBER PRESSURES.
Of the two methods, (1) is more work, but is much preferred, as it is safer. Case form and load as described, and the case will be properly and safely supported for that first firing.
HTH
John