[Exim] Exim and Sendmail

Hello,

I'm trying to make a smooth switch from sendmail to exim.

My concern is that I don't want to switch all my users at the same time.
I'm afraid of something going wrong and I want some of them to have
tested everything is OK before I switch the others.

In that purpose, may I safely run both systems on different ports ?
Does each of them manage compatible locking mechanisms to prevent
incoming mail jam in the mailbox files ?
Do I have any care to take when configuring systems ?

Thanks for any help

Jacques