So you want to implement a new feature, and test it locally, then run tests to make sure you didn't break anything, and when you are done, upload the changes to the production server. This is sane devops, nothing fancy, yet impossible with todays databases. You basically have to manually do the same changes you did in development to the production database, then test if it works on live data in production. Then your finger might slip when writing an SQL query and you have to reset all production data from backup.
Anywhere between maybe 1-2 man hours and multiple man months, depending on how nice the data is right now, and how accurate it need to be split. The optimal case would be the application already enforcing values to look like e.g. "last, first", so that the migration can happen via a simple regex.
