Take a look at replace field contents with the calculation option. This tool can modify a field in every record in your found set in one go.
Here's a sample expression that would ad "AD" to every record in the found set:
Replace Field Contents [YourTable::yourTextField ; YourTable::yourTextField & " AD" ]
You'd need to perform a find to pull up only AD dated records first before you do this operation.
Since it can change a great many records all in one go, make a back up copy of your file just before you try it so you can start over if you don't get it exactly right on the first try.