Recommended instalation steps

Do you specifically want to end up with just 2 servers or are you intending to keep all three?

If you want to end up with two servers, I would do it as follows:

Build your new server as a BDC, but assign it a permanent name - you don't want to have to play around with renaming a live Exchange server.

Next, ensure that you new server is fully prepped with the latest service packs, etc...

Install Exchange on the new server and ensure that you join it to the existing organisation and service pack it to the same level as your main Exchange server

Migrate mailboxes across to the new server using the move mailbox wizard.  You're best off doing these a few at a time to minimise disruption to your users.  Apart from the downtime during the actual mailbox move, which you can do out of hours, the change should be seamless

Replicate Public Folders across to your new server.  Again, do a few at a time.  If you select all of the public fodlers at once, you'll get a nasty shock as they're hidden from view until initial replication has taken place.  After a few days, you can change the home server for the public folders and stop them from replicating back to the original server

Move any connectors, such as your SMTP connector across to the new box.  If you're using DNS to send & receive SMTP mail and you don't host your own DNS, you'll want to get your ISP to add anothher and A and MX record pointing to your new SMTP gateway.  Give it a couple of days and then create the IMS on the new box.  Then, when you're happy, modify the address space properties of the IMS on each server so that thhe new IMS is thhe preferred one.

then, read the following article for the final steps you need to perform.

http://support.microsoft.com/default.aspx?scid=kb;en-us;Q152959

Once you have removed Exchange and any other necessary service from the PDC and ensured that all data is safely moved elsewhere, shutdown the PDC and promote the new BDC to PDC.

The old PC (MainServer) will die.
If I assign a permanent name to the new Server, the users must configure their mailboxes with this new name rigth ?
On the other hand I must change the server in 1 day.
What Do you think about this.
Install the new server as BDC, shutdown the MainServer, promote the BDC to PDC, change the PC name as the oldest MainServer , and then Install Exchange.
Is this procedure ok ?, If it is , How do I restore the mailboxes ?
Thanks again

No, the idea is that you would be adding a 2nd Exchange server to your Exchange site, migrating the mailboxes, public folders, connector, offline address book, etc.. across to the new box then removing Exchange from the old server, deleting it from the site, shutting down the machine and then promoting your new box to PDC in its place.

It's a much neater process and there's nothing to stop you doing it in a day, but unlike relying on backups, etc... you always have a clear pathway to back out of the changes if you are unsure at any point.

thanks hstiles for your answer.
I will do the procedure as you suggest.

So if I add the 2nd Exchange server to the site I can use the same name ?.
Sorry If I insist, this is my first experience with this.

I forget to told you that we are going to change 3 servers.

Server A with site connector to B and C
Server B with site connecot to A
Server C with site connector to A

In which order do you suggest to make the change ?

Thanks

You said that the new exchnage server must have the same Service pack level as the old one. Is this necessary ?
My old Exchange has SP 3 and I want to install SP 4.

You said that the new exchnage server must have the same Service pack level as the old one. Is this necessary ?
My old Exchange has SP 3 and I want to install SP 4.