6mm Creedmoor to 6.5 Creedmoor

It’s called the case neck. The throat is inside your neck. Yes, it’s ok to do that, just use plenty of lube and expect increased seating pressure afterwards for the first firing cycle.
 
I do 243 to 260 all the time.
Get a mandrel (.262ish) and die, lube with imperial.
There’s a possibility of donuts to be aware of.

You can two step it with a .257 mandrel first, reducing force needed.