If statement with 1 condition as no change
I have a blank field that has a number imported in from another file. The past numbers in this database have been correct, however now it's not and any new records will import a new number that I want to go into that blank field, which is done by a set field step in a script. But if I mess with the current field and have a new number imported I will lose all the old data, correct? Because the field will be defined differently and the old records will not be able to find the new imported data, which doesn't exist in their records. What is the best way to deal with this?