How to update the old openldap to the current version, keeping the database and all settings?

Question

linux

Hello!
The old openldap version ( slapd 2.4.15 (Mar 19 2009 10:07:59) ) is on the old ubuntu server 9 , everything is configured for it in the LAN.
As I understood from the articles, it used to be (just 10 years ago) that it was fashionable to make a config on Samba 3 and OpenLDAP, and now many people just use Samba. Therefore, raise the server to Samba 4 AD and somehow transfer at least information about current users to it so that people can log in to their domain accounts without creating them again (even without saving passwords).
Really, there are 15 machines in the company (it shrank 10 times), I want to do something simple both for the simplicity of the network and for self-learning. Only I stupidly do not know how to transfer accounts from one domain to another.
My task is to transfer this entire server to the current ubuntu 18 with the current (there was a 2.4.45+dfsg-1ubuntu1.4 package in the repository) openpldap. The problem is that the old one has other config files. Moreover, the new one is very different from the old one, it is sharpened for customization in phpldapadmin.
At the moment, I have deployed ubuntu server 18 on a virtual machine, put the current openldap on it, and I don’t know what’s next.

Answer 1

What's next? Further - handles to transfer configs.
Jokes aside, this is better.
And this, in the yard, consider October 2019, install Ubuntu 19.10 better.

Answer 2

So, tired of hitting the wall. And if this option:
As I understood from the articles, it used to be (just 10 years ago) that it was fashionable to make a config on Samba 3 and OpenLDAP, and now many people just use Samba. Therefore, raise the server to Samba 4 AD and somehow transfer at least information about current users to it, so that people can log in to their domain accounts without creating them again. (even if WITHOUT saving passwords)
Really, there are 15 machines in the company (it shrank 10 times), I want to do something simple both for the simplicity of the network and for self-learning. Only I stupidly do not know how to transfer accounts from one domain to another.