Migrating code from 4.0 to 4.2

We have a requirement of moving code from 4.0 to 4.2, 4.2 is already installed and we have jobs running on that smoothly.

Please let me know what should be the steps, best practice and check list.

Will importing the ATL file be enough ?

Also let me know if there is any thread related to this.

Thanks

Sandeep