Hi Truus,
Yes, it is possible. To switch from a test center appointment to an OnVue appointment, you must first cancel their test center appointment and reschedule a new appointment selecting
the online proctoring option as the test delivery experience. If you initially paid with a voucher, the same voucher should be reinstated as valid and you should be able to use it again for the new appointment.
Please refer the link given below for more reference.
https://www.microsoft.com/en-us/learning/community-blog-post.aspx?BlogId=8&Id=375289
Please let us know if you need any further help, we would be happy to help!
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Veena
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