Booking Status: Field Service Status can’t be changed. Record is already used on Resource Bookings

While using Field Service there might be the situation where you figure out that the Field Service Status does not fit to the booking status, or the Field Service Status was not provided at all.
But if you try to change the Field Service Status field on a booking status record that was already assigned to any resource booking in the system you will usually see this error coming up:

If the system only has a handful of resource bookings it can be handled easily in a manual manner by temporarily switching the booking status to another status.
But consider any automatic workflows or business process that could be behind any of the status changes.

You need to make sure that those will not become triggered by a temporary status change! (E.g. service report generation, etc.)
Please note:    
Be aware that the booking status on the resource booking affects the system status of the assigned work order. E.g. if all bookings have status completed the work order will change to Open – Completed. Applying the fix below will temporarily circumvent the default behavior of changing the booking and work order status.

To work around the error above… there is another possibility to temporarily disable the Field Service bookingstatus plugin on the update (pre-validate) event.

Then change the Field Service Status to the required value and re-enable the plugin again.

Original Post https://code2life.blogspot.com/2018/11/booking-status-field-service-status.html