AD: Windows 2008R2 to 2019 migration, one leap or step by step

If you want to upgrade the actual servers by performing an in-place upgrade (which I highly advise against), you need to go through multiple steps.

But if you want to add new 2019 Domain Controllers to replace the existing ones, then yes, you can definitely add 2019 DCs in a 2008R2 domain, after the standard preparatory steps (adprep).

Also make sure your SYSVOL uses DFS replication instead of FRS; which it should, but sometimes FRS just stays there because nobody bothered changing that after upgrading from 2003. FRS is not supported anymore and you can't add a 2019 DC in domain which still uses it.

Also, take extra care if you have non-Windows (or old Windows such as XP) devices that use Active Directory for authentication; they might be not compatible with 2019 DCs due to their stronger security settings.