Bad news first - It is rare to re-chamber a rifle with a smaller chamber, as one has to cut the barrel back a bit, and then re-chamber it, whereas when you move up in case size you only have to bore it out.
Good news, unless I am mistaken, the 300 Wby is basically an improved 300 H&H, and as such, it is safe to fire 300 H&H cartridges in a 300 WBY chambered rifle. I would check with someone smarter than myself first, but I know of others who have done this with no other ill effects than a slight loss in velocity.