Hi @Rahamadulla Abdul Rahman ,
it depends on the Management Pack. If it is a custom solution, depenidng on how it has been developed, you might be able to use it in your new SCSM 2019 Management Group. You can read more here:
SCSM 2016 – Upgrade steps for custom development
The SCSM native specific Management Packs for SCSM 2019 have their owne version though so you cannot take those from an existing SCSM 2012 environment.
I hope I was able to answer your question.
----------
(If the reply was helpful please don't forget to upvote or accept as answer, thank you)
Stoyan