If you want to be able to latter, at any time, to restore original values without resorting to the use of an older version:
1) You can use formulas. Use one custom field to save actual values using a formula. Use another formula to set all values equal to productivity. Then by running at will your formulas you can switch back and forth.
2) You can use Standard Reference Books, create a Standard Reference Book for Activities with the model as is make a copy to another version of your new Reference Book and apply your changes the way you want and then whenever you want you can restore your values. Be careful this is a powerful option that might restore other values. Recommend deleting from your Reference Book all fields except Activity Code and DPH Type. You can apply formulas to reference Books independently of your model. Once you learn how to use Reference books you will be hooked, it is more powerful than import/export to Excel.
Start with formulas, and then try with reference books as a learning exercise.
The following are Reference Books Screen print out.
After that you pay the Coronas.