You might want to start here..at Upgrading, but there are a couple of other docs that are relevant.
I am not sure of the state of play with RHEL, but it should be OK, as long as PHP and one of the four appropriate DBMS' are installed. I do not think there are any serious issues, but an alternative might be to upgrade on your Debian server to the last v1.9.x release first, then backup and restore this over to the RHEL server before upgrading. Make sure it works by editing the config.php file to reflect the new server location, and if you need to, use the replace.php file to update any old server strings restored. I think the last v1.9 releases were slightly restructured to allow for an easier upgrade to v2.x - or they should have been at least.