You may change your massage service upon check in, but the duration of the massage service has to be either shorter or equivalent to the massage duration that was booked. Change to a longer massage service will be subjected to massage room availability as well as therapist availability.
If you are unsatisfied with your assigned therapist, you may request for a change of therapist within 15 minutes of starting your massage service (subjected to therapist availability), however, once 15 minutes has passed, you will not be able to change to another therapist for the remaining duration of the massage service.