Sales order to retrieve Unit price based on Adv pricing but subgroup item ctg codes

On a Sales order when an item is entered it retrieves the Unit price based on the item category code price. Is there a way not to use the item category code to calculate the price of an item for certain items to certain customers. We want the customer to still get the same price that he was charged on 1st Nov. So, if an item ctg code has changed at say on 15th Nov, the price now is from the new ctg code. Can we use a Contract price where it retireive a certain price till the eff