Based on my understanding , the DCs in your domain are based on the Server 2008(x86) ,now you want to use the server 2019, and raise the function level ,right?
Before going further , i would suggest you back up the old DC.
Make sure the function level for you domain is 2008 ,and if the DFSR is used for the sysvol replication.
command :dfsrmig.exe /getglobalstate
If the Result: 3 (ELIMINATED) the DFSR is used for the replication.
If not , you need to migrate the FRS TO DFSR before promote the first 2019 server. For more information , you can refer to :
After the DFSR migration , make sure the old DCs are working well and no problem with the replication, you can promote the first 2019 DC.
Confirm everything works well the command:
Dcdiag /v >c:\dcdiag1.log
Repadmin /showrepl >C:\repl.txt
Transfer the FSMO role from the old DC to the new one.
Demote the DC.
And then you can try to raise the function level (domain function level and forest function level) to 2016.