If you sell something, the price today is set in the invoicelineitem price field, not with a calculaiton, but a lookup.
Lookups CAN be updated and the data changed, but with correct trigger choice, only by intent.•If you change data in theof the related or in the source field for a lookup, FileMaker Pro doesn't automatically update the data in the lookup destination field. To update the data, you must look it up again. This occurs when the value in the match field changes in the lookup destination table, causing FileMaker Pro to perform a relookup.
A suggestion that can help you transition from a calculation field to a lookup field without losing your existing data:
Open manage | database | Fields, find your calculation field and change it from type Calculation to type Number. The current, stored value--assuming that it is a stored calculation, will be retained in the field, but it cannot now recalculate.
If this is an unstored calculation, You can define a new number field and use Replace Field Contents with the calculation option to copy the current value of the field into the new number field.
With both of these options, make a back up copy before making such a change just in case you don't get the results that you expected.