We understand that a migration is often a process with multiple phases. This is why we recommend completing your initial migration well ahead of time - this is to move the mass amount of the data. Then once the data has migrated across, it's just a case of transferring the modified and new files from the source to the destination.
To support this, we have the 'delta' option under the settings of each migration. This will compare the modified times on the source to the destination and if anything is newer at the source, we will transfer those files across. Keep in mind we don't remove the files from the source, simply think of it as a "copy and paste" to your new cloud platform.
Enabling the delta option
When configuring your migration, under the settings you will see the option of delta enabled, simply tick this box and either save and kickoff or save for later, when your next migration runs it will only moved the modified and new files.
Configuring delta sync:
For more information on delta migrations see our blog post here.