A had a major setback last weekend on my "simple project" of replacing the barrel nut on an older Savage 110E 7mmRemMag. After getting the proper equipment for the job (Barrel nut wrench and barrel vise) I discovered that the barrel is larger than the barrel nut.

When I started unscrewing the barrel nut and found that it was digging into the unthreaded portion of the barrel (this was not supposed to happen according to directions I found on Brownell's website). SO I completely removed the barrel from the action (with ease).

Now to the real question. IF I want to change the caliber of the rifle to .25-06 (since I two 7mmRemMags already), would it require a new bolt assembly or a new bolt head?

To change from a 7mm mag to 25-06 would require a bolt head change. Try a case in the magazine to see if maybe you might need a new follower also.

Check also that the barrel nut is still round. Sometimes they get bent and dig into the barrel.

Here's a link to the Savage Shooters forum. It has more info than you will ever need, on changing barrels and bolt faces.
Savage Shooters Forum

Nashcat