Well you could always trade it in for a Savage and a good scope!!!
Just kidding. Well here are my thoughts. You have a great platform to learn off of. The big thing this rifle will do is give you a lot of trigger time for a lot less and teach you the fundamentals of long range shooting and hunting. A 400yrd shot is a drop in the bucket for the .308. This caliber will help you find any inconsistencies that may occur under recoil. Being new to the long range game requires a lot of trigger time and learning to manage recoil. This is also a great way to learn how to read wind and learning your distances.
Unless you are use to shooting heavy calibers, the 300WM may develop some unwanted bad habits. You might start flinching and relying on the caliber to do the job and not proper shot placement. I don't know what your hunting experience is, but you may already know this. The 300WM would definitely be the better option for long range hunting. You can load it up with heavier bullets providing better ballistics down range. If you are use to, or have shot heavy calibers and recoil is not a concern, then maybe the upgrade is an option. But, if you are buying loaded ammo, then your shooting cost will be higher, the wait between shots will be longer when the barrel heats up quicker, and recoil management is a lot higher of a priority.
I think starting with the .308 is a great option. If anything, you could do what I did and have the chamber reamed 300WSM or 300WM when you are ready to upgrade to a larger caliber.