Multiple Find and replace

Hi
Can anyone help me, I need to recode 30+ files the have the old account numbers and I need to replace them with new codes ie

Old New
123456 10_1240
256789 12_1130
789123 15_1122

I have an excel spreadsheet which contains some 800 accounts in the format above showing old account number in column A and the new account number in column B. How would I write the VBA code to search all files in a specified folder search all rows in column E and find/replace using my spreadsheet containing the mapping details as above?
or in the very least just be able to run a macro to replace all the old codes in an individual spreadsheet.

Thanks in advance for you help!
Sarah