when i make an admin backup of his old account and restore it on the new, what will happen to some data present under his account on the new server?
Will get overwritten indeed.
However I didn't know this would also delete all files when only doing a database admidn
But on a new server, which data would you already have on the new server? Except nameservers maybe?
This is how I normally work.
1.) Create new server and setup
2.) Restore the old admin account (will overwrite the existing one)
3.) Adjust admin account as needed like nameservers for example.
4.) First restore all reseller accounts (if present) then user accounts
5.) change all NS to new ip's on registrars if needed.
Another option you can do is to rename the admin backup.
Normally it will be admin.root.admin.tgz if I'm not mistaken. You could rename that for example to admin.root.admin2.tgz for exmapleand then it will be restored as admin2. Then choose how you want to go furter.
Personally I find it easier to just overwrite the admin account at the start of installing, then the rest is easier.
But it would be good to have a "keep current files" option indeed.