Necking down is easy to do. As Nightstalker said there are some considerations. Anealing will help as it softens the neck. ID will be controlled by the expander. You may get excessively thick necks that can contribute to higher pressures. Compare the thickness of a factory .280 case with the newly reformed case. You may need to thin the area. Many prefer to turn the OD to get the appropriate thickness. Trimming will be essential also.
Even though the newly formed case looks just like a .280, it has some significant differences. You MUST treat it like a very different component and work up a load for that case. Remember, different makers have different thicknesses and require different loadings.
My advise would be to take a good sized batch of ONE headstamp (R-P .30-06 for example). Size them down. Trim. Ream or turn the necks. Flashhole debur. Clean and uniform the primer pockets and use them separately from other brass.
On a couple of side notes;
I have found the one operation you can do that seems to most influence accuracy is flash hole deburring.
Winchester brass seems to have the smallest primer pockets and really needs to be uniformed. R-P does not seem to benefit from this operation.