Hi Sudarshan
I have tested your scenario in my system. I am little surprised that config doesnt allow me to assign same valuation class to 2 different AREF (Account reference number/Account category reference).
Can you please check entries in OMSK and see if 1200 is linked to ZPQR or ZABC or both???
Can you also check the exititng ZABC material type with 1200 val class that what has happened to them when you made the change?
As far as impacts are concerned, I am assuming that not much has happened since you created new material type in terms of purchasing/postings then its fine to move the 1200 to ZABC via OMSK