How do I abstract a WHERE clause condition?
OK 5 years later, here's a possible solution.
If this is your database and you are allowed to change its structure, I would

Create 2 new fields; OldC and NewC.
Update all OldC values with the value of ColC.
When ready to calculate, update all NewC values with the new value.
When ready to migrate, update all ColC with NewC.

If during this time, you need to know whether ColC is the old or new value, compare it to the 2 new fields.