It looks like my tenner is safe, unless Kodak have got it all wrong.(posted 8530 days ago)
Here's what they say in one of their technical publications on grain and granularity."Development usually does not change the position of a grain,.....", and again in the caption to Fig. 22. - "Contrary to widely held opinion, there is little migration or physical joining of individual grains."
I hope this lays one 'well known' myth to rest, since I could find absolutely no contrary reference to grain migration in photographic film at all.