My first question would be, how does the 300 weatherby shoot? If it shoots decent now, and has at least half barrel life left, then I would not touch it. Leave it alone as a back up. The reason I say this is that your not going to turn it into a 338 Laupa for pennies. Now I know that it may be cheaper than a whole new rig but for the extra money you'll have two guns instead of one. Also, I don't particularly care for the weatherby rifles myself, I just haven't had good luck with them. Just MHO, don't want to start a war with anyone here! The best thing to do would be to speak with one of the more knowlegable guys on the site to weigh the options out cost wise and go from there.
you cant dump'em if you don't pull the trigger!