For the increase in usefulness (which I believe to be small) I would not convert it if it were mine.
But that's just my judgment. I hunt elk every year and the 35 Whelen is fairly popular here in Wyoming for elk and moose. I have seen several animals killed with it, and I am very impressed at how well it works without a lot of recoil, and with a 5 round mag and 22 inch barrel.

You'd need to either open up the bolt face or change the bolt to a magnum bolt. If you open it up, you need a new extractor (no big deal)
Next you will have to re-chamber the barrel.
Next you will probably have to tinker with the magazine box a bit to get it to feed smoothly.
When you are all done you will gain some velocity.
But to put it in proper perspective we need to look at the velocities over a bit of range.

The 35 Whelen will start a 250 grain bullet at 2400 FPS and the 358 Norma at 2700
That's quite a jump, but if you look at the figures at 100 yards it gets more reveling.
The Norma is going 2400 at 100 yds, the same as the Whelen is at the muzzle.
So there's no question that it will shoot flatter, and have the advantage at every range mathematically.
However the "real world question" is this:
Is that extra 100 yds of range over the 35 Whelen of any real value in the game fields you will hunt in?
That's a question only you can answer.
And then, if the answer is "yes" you must decide if that advantage is worth the expense of the conversion.

I wonder if my statements here have helped or hindered you.
???
I hope it was helpful

Happy hunting
Steve Zihn