No - switching servers will not result in you losing your existing cities.
It is possible to create up to 10 regions in SimCity, each with multiple cities, across each of the out of game regions that servers are hosted in (Europe West, East, Oceania etc). Each of these game servers are standalone from each other, so the first time you log into each server you will need to do the tutorial and create a new region to play in.
Games on each server are only available on that server, however if you change to another server and later return to your original server all of your saved regions will still be there.
For example, last night I created a save on Europe East but then was disconnected and was unable to get back on. When I logged into the Oceania server I was able to log in but had to complete the tutorial again, and then create a new region and start from scratch. Later on I tried to reconnect to the Europe East server and my original game was saved and waiting for my return.