I'm not sure if there is a script that will merge everything but you could take the imported new Sarah Brown account and rename it something totally different, then update the Sarah Smith account to be Sarah Brown. Then the import should use whatever target key you have set up and begin updating the right account. Then just delete the imported, renamed Sarah Brown account.
You could also use something different from your import that doesn't change in a case like this like an Employee ID number or something similar to be the primary key instead of login name or email address.
Hope this helps,
I didn't think to use the Employee ID as the primary key. That makes a lot of sense. If someone does know a way to merge, that would still be appreciated, as we do have users who currently have incidents related to both accounts. I will see about changing our primary key for the future.