Is there an "undo" for list imports?

Currently there is no way to undo a list import, but in my opinion there should be. The support team at Marketo is usually able to provide you the data as it appeared the day before the import which you can then use to import and reverse the data back to the way it appeared before the upload.

You might also set up a flow action with a smart list "is in filename" and change the data values in both fields.

This tip is ill-timed given your current circumstance, but we export all columns of our full Marketo database each month and it has helped us restore several records that were corrupted.

If you have a limited number of Lead Status values, you can create a smart list with the Data Value Changed filter and look for changes from each Lead Status value to "unsubscribe" on the day the change was made.  Then select all of the leads in the smart list and use the Change Data Value flow action to change the value back to the previous value.

Would love to see an undo implemented.

Elliott's suggestion sounds good. We're going to have more Marketo users in the company and I know that more people will be importing lists, so having the undo functionality after importing a list incorrectly would be really good. 

Love your suggestion Elliot! Never thought to do that. Genius, I say!