to me an agent reinstall means pushing any agent install to a machine that currently has an agent. I prefer self contained agent pushes which sends the full agent install at a device in a exe and has worked well in our ldms upgrades. we do controlled rollouts too, usually give a week to push to all our machines. our current self contained agents are 56mb. as you deploy more agents from the new core server, managed devices will be migrated from the old core to the new core.
hope that helps.
Rob