Implementing new AV on Network

hiya ppls :)

just a question, we are moving from one av solution to another one, but we first we have a grace period of 6 mnths testing of the new av. my question is this.
what would be the best way to implement new av...where do u start?we have about 10 servers +- 5000 users. do we start with a server and set amount of users, or per dept? what would be the best way to start someting like this? and any tips on testing the new av?

thanks alot :)