if you have stock, then this stock is revaluated with the new price and the difference is posted to G/L account.
if you change the price in a previous period, then you have to change the price twice, for the previous period and for the current period, otherwise you get a second revaluation back to the current price
if you have a cost estimate, then you receive error CKPRCH025
Read OSS Note 1122813 - MR21: Error CKPRCH 025
it explains to change the message attribute, user specifc, in Customizing for Materials Management invoice verification (OKZZ), change error message M8 155 to an information message. You can then change the price.