Mass Updates - Keep Users Out of System While Executing?

We are going live with a major new AgileApps application on Monday morning.

We will have to do some Mass Updates of the imported data. For example, we know that for 3 of the users we will need to change their Ownership of records.

Is it best practice to have all users logged out of the system in order to make sure there are not data conflicts on records? Such as when someone is editing a record, and then a Mass Update is made that changes a data value on the record - what happens to the record in that situation?