I generally recommend migrating (backing up settings & data) with a clean install and re-integrating data (although not OD data where possible), when moving from any one major version (of really any OS) to another.
First, please read the following Apple article:
Now let’s ensure that you’re using the Apple article as intended. In the Terminal on your server (or logged in via ssh):
sudo mkpassdb -dump | grep diradmin
where your OD admin account shortname is indeed diradmin
The above command will probably come back with
slot 0001: 0x00000000000000000000000000000001
if not then note the difference, specifically the long string (0x … etc.).
Then issue – adjusting to match your slot ID if it differs:
sudo mkpassdb -setpassword 0x00000000000000000000000000000001
If it’s been more than 5 minutes (by default), do keep in mind that the first password you’re asked for will be to authenticate as the local admin for use of “sudo”