If a Foundation Dentist (FD) has started their tenure, you cannot edit their Vocational Trainer (VT) on Compass.

You should make the FD part time, with an end date the same as their start date and amend the financial values to zero by clicking 'Edit' on the FD. 

The FD can now be added under the correct trainer, but because the entries overlap for one day, you will need to make them part time but then edit the financial information to enter the full time values if appropriate.