Unfortunately it isn’t that simple 
Partial switches tend to be the most unreliable as although the banks contact the Service User (merchant), they don’t always update their records and set up a new instruction with us. As the bank you switched the instruction from will have cancelled their instruction, any DDs to the old instruction will fail.
Equally, we can’t switch a salary payment (Direct Credit) in a partial switch - this is mainly done by payment redirection which is a ‘feature’ of the full switch.
The Current Account Switching Service also has a few rules around how the options have to be displayed, and some very strict SLAs for actually completing switches.