I think you're going to find that most of those newly retired BA 747-400s are
very high hours and have earned their retirement - as in north of 100,000 flight hours. Hence I doubt many (if any) will become freighter conversions - it's just not cost effective to spend all that money turning an already very high time aircraft into a freighter. Their future will be as beer cans
The 747 will be around as a freighter for several more decades - the only real question now is what will happen after the dozen or so new production 747-8Fs currently under contract are delivered. Will Boeing pull the plug (as suggested at the beginning of the the thread), or will there be sufficient interest in new build 747-8Fs for Boeing to invest in new fuselage manufacturing capabilities.