Hi Babu,
You are right - MR21 changes moving average price, and if the article has stock then there is a P&L impact.
Do you have CO-PA in your system? If so, then you can tick a checkbox in OKB9 (instead of directly filling any cost center) and it will derive profitability segment characteristics automatically based on other available information (material number, plant, profit center, material type etc.).
If you need a cost center etc. which changes based on the above then you will need a CO substitution (trans. OKC9) but for this, you will most likely require a user exit (determining the cost object based on other data in the item).
Good luck!