You can easily transfer one user's progress into another on the Users page. Click the three dots on the right of the screen and choose "Transfer progress".
This will take you to the transfer page where you can select two users.
Decide which credentials you want to keep. Progress from the other account will be transferred to the selected account. The other account will be removed. Confirm by clicking the "transfer progress" button on the bottom of the page. And voilà, the magic happens.
Please note: This action does NOT merge the users' progression. The progress of the user that you keep the credentials of will be overwritten by the transfer.
Example use case
You can transfer the progress of learners who started their pre-boarding with their personal email addresses to their work email addresses once they start the onboarding program after contract start date.