Configuring MX record switch over to hosted Exchange

The idea of what we want to do is have our Exchange server be the primary host of our mail. We also want to keep InMotion as a backup host in case our server goes down.

So would we have to do the following..

1. In MX Entry in CPanel, change the radio button to Backup Exchange.
2. Add the Exchange server MX record and give it a priortiy of 0
3. Change the Inmotion MX record to a priority of 10

4. Go into DNS Manager in our Domain Controller and change our MX record to priority of 0.
5. Add the InMotion MX record and make that a priority of 10.

We also have a firewall that we store our pointer records on. Do we have to add MX on there too?

I'm also assuming that I should do this on a Friday night and that it could take up to 48 hours to complete?