Upgrading is not advisable, but you're talking about migrating.
So doing a new Centos 7 installation an a new VPS and when finished, installing Directadmin on there and migrate your users. This can be done via admin backup/transfer.
You want to know if you will have any problem or loose users.
You won't loose users normally when transferring them all to the new VPS. As said, admin backup/transfer is normally a preferred way to do this.
However, things can change. To prevent certain issues occuring, it's best you match versions of what you're using on the old and new vps from Mysql/MariaDB and PHP.
You can install a newer version of php, but older scripts of customers could loose their functionality then, so check this before.
It's also possible to migrate the users from the old server Mysql to new vps with Mariadb, but you have to check versions!!! For example, if the old VPS is running mysql 5.7, it's wise to have MariaDB 10.2 running on the new server.
Most safe way of migrating is to have MySQL and PHP versions the same on the old an new VPS and if needed convert or upgrade them afterwards.
