I'm so confused!!

I am currently getting ESA WRAG and received a reassessment form last November. I filled in the form and sent it off well before the cut-off date. I heard nothing from them until this week when I received a letter from the health assessment advisory service informing me I have a telephone assessment with a health care professional next Wednesday. This I was expecting, although it has triggered my anxiety.
My concern is that last week I received a migration to UC notice, something I have been dreading for years. With the help of my sister-in-law, I applied for UC as was explained on the migration notice. My anxiety levels have been insane since I got the migration notice, I have to prove my identity over the phone tomorrow.
I'm now in a tailspin because I'm confused about the ESA assessment?! How will that affect my UC claim? Will the migration go ahead? Will it be seen as a new ESA claim? Will the ESA assessment be cancelled because of the UC application?
I have a brain injury, CPTSD and am dyslexic, I am so confused, terrified and beside myself with worry. I just don't trust the system and am terrified they will fluff it all up.
Can someone help me understand it all?
Thanks

Thank you for that info, David.
However, I'm still confused about why I need an ESA medical assessment if I'm being migrated to UC. Will the outcome of the ESA assessment affect the UC claim?
I'm so worried they will deem me fit for work (even though not a lot has changed in the last 10 years that I have received ESA)
Thanks again