You can swap, yes; if you wanted to go 35 Whelen. Barrel & Bolt head. You would need to remove the Fixed Ejector. And I’m not sure about the mag box. It’ll fit… and f that I’m sure. 375 H&H one BIG BOY next to a 30-06 case. But why the sudden interest to switch? Especially to what’s known as the Poor Man’s 375 H&H, in the 35 Whelen? Not that it’s a bad cartridge.. but if you already have the H&H? Is it recoil? You can always change it to a 300 H&H. Or better yet, 300 WinMag.

What is it you are after? I ask because you mention “keeping your options open”, and you mention quite a swing from 7mm Rem Mag to 30-06 to 35 Whelen… What is driving this thought that you need to change up? And especially, take on the added complexity of changing parent cases?