I would send a UC Journal message and choose from Recipients A Payment which means that your message will be routed to the UC Case Manager. All you have to say is that - I was in receipt of payments of ESA at the date of Managed Migration so my WRAG/LCW status by law is carried across to my new UC claim and a Fit Note is not required.
Let me know how you get on. David
Nothing on this board constitutes legal advice - always consult a professional about specific problems