Two mail servers on the same domain

[Dusan]

Dear all,
this is a quite newbie question, I think :)
We have an old mail server with about 200 users mail accounts.
We want to exchange it for the new one, and we are planning to use
following method, which we don't know if it would be correct.
We want to install a new machine on the same domain and to configure
mail there (postfix).
For example a user John Thomas has an account "jthomas" on the old
mail server. We would make a new account "johnthomas" on the new
running mail server.
In the DNS server we would have 
ourdomain.sk  IN  MX 10 mailold.ourdomain.sk.
              IN  MX 20 mailnew.ourdoamin.sk.
So our reasoning is that in the case somebody sends an email to
jthomas at ourdomain.sk it will go directly to maiold and when sending to
johnthomas at ourdomain.sk it will go to the mailnew.
By this method we would create all accounts in the new manner...
Is this correct ?
And in the case it is not, how this should be done ?
Thank in advance for your advice
Regards
Dusan