serial number < 101 ; YourCurrentEquation ;
"So, I'm wondering if it's possible to change the equation without affecting the existing value?"
If you need the field to hold the value to maintain history then it should not be a calculation but rather a standard field. If this is correct then you have two considerations ... what to do with existing data and what to do with future data. This is often used with Invoices where the customer address might change over time but the address at the time of shipment must be preserved.
Hard-coding by the serial number < 101 doesn't improve the situation - it only hard-codes by a different criteria as you are discovering, Leon. The solution is to plant the data into the standard field at the time it is called and then not change it. But you didn't provide the calculation so we don't know if the calculation uses related fields, global fields, other unstored calculations ... and a proper response will depend upon whether the calculation can be indexed.
Auto-enter by calculation (leave checked 'do not replace existing value') or even lookup will handle all future records. But FIRST, you will need to clean up the records you have.