Hi,

This is a bit tricky. But here's what I would do:

1. Back up your database.
2. Zip up your current DNN installation (all folders).
3. On the new hosting environment, create a new server and restore the DB from the backup.
4. Unzip your zip file from step 2 into the appropriate folder (domain root or subdirectory, depending on hosting environment.
5. Modify the connection string in the web.config to point to the new DB.
6. Make all necessary settings on the IIS, and make sure you give your folders and DB all necessary rights (there's plenty of documentation around on how to do that, some of it depends on your hosting environment).
7. Copy the machine keys from your old web.config into the new web.config, or nobody can log in any more.
8. Run.
9. Now, login as host (if you can't log in, make sure you've copied the right machine keys from the old web.config!), navigate to Host > Host Settings > Host Details and at Host Portal select the new main portal from the dropdown.
10. Navigate to Host > Portals and remove all other portals.
11. Navigate to Admin > Portal Settings and remove all other Portal Aliases.
12. Repeat this process for each portal.
13. When you're done restoring each portal in its own new home, remove all unnecessary portals from your own installation, (if you want to continue running your own site), including all portal aliases. Modify your own IIS to remove all host headers that you no longer need.

I'm not guaranteeing this works, but I'd give it a try. Since you're not making any changes to your own website (with all its portals) until after you've successfully migrated the "children" there's not too much risk there. (Except the risk of needing a lot of time to get it all done)

However, if it's at all possible (with regard to modules used, esp. third-party modules), I'd upgrade your whole installation to a more recent version of DNN first (4.5.5 is very stable) first. That way, each of your new portals will also automatically be up-to-date. I think that would be a nice last bit of service  for your clients! Make sure you read the available documentation on how to upgrade DNN though, especially with regard to changing IIS settings and the web.config!

Hope this helps.

Vera