I see you basically have the answer you were looking for =)

But I thought I would reply to this (because it's a good question) ...
Also, if it was "only" switching mail servers why would I need Lion?

... with the answer that iCloud is more than just Mail synching and I think they rolled-out the package of iCloud capabilities with Lion in order to avoid the comparative logistical nightmare of offering individual "updates" a la carte for varying versions of OSX =)