[stackskipton]

Former Exchange Admin here: 1 is easy, I used to do 70k mailboxes in middle of the day only but it requires spare hardware or virtualization with headroom.

Deploy new Server(s), patch, install Exchange, Setup DAGs, migrate everyone mailbox, swing load balancer over to new servers, uninstall Exchange from old, remove old from Active Directory, delete servers.

BTW, Upgrades now suck because Office365 uses method above so upgrade system never gets good Q&A from them.

[EvanAnderson]

Same feeling here re: migrations being easy if the Customer isn't a cheapass. Small business Customers who had the competing requirements of spending as little money as possible and having as much uptime as possible were the stressor.