Very interesting question. It seems that the increase in partakers forced a change in doctrine, rather than the other way around.
One thing I have come to realise is that a heavenly resurrection is a more attractive belief than an earthly one. Fpr a JW if the end is not about to come any time soon it means at death you are gone and just rotting in a grave. Whereas if you believe you are going to heaven you get to never be non-existent but go straight to heaven.
Another point is that being of the anointed holds a title of prestige. It is interesting that recent articles state that being of the anointed is not to be taken as being of more import. It is as if the GB are trying to stop people choosing to be so for reasons of title.
Watchtower 2009 June 15 Page 23 : "However, Christians who have truly received this anointing do not demand special attention. They do not believe that their being of the anointed gives them special insights beyond what even some experienced members of the "great crowd" may have."
I have an article on the increasing memorial numbers at http://www.jwfacts.com/watchtower/memorial-partakers.php