In order to change a student's SEN Provision you will need to end their previous SEN Provision, then enter a new one. On the student's profile, on their Support Tab click on the cog next to SEN and Edit against the current Provision.
Place an end date on the SEN Provision, and on the SEN Need at the base of the entry. This will then go into SEN History as a record of the student's SEN provisions. A table will come up where you have the option to Add New, and you can enter a new SEN Provision and new SEN Need associated with it.
The new SEN Provision will then appear in the table, with the start date given.