Swap to SIM only when contract ends.

My 2 year contract on buying a new phone runs out on 8th March 2020.

I want to swap to a 30 day SIM only contract @ £5,  - but there is no option given in ‘My Account’. There are options for other more expensive contracts, but not for the SIM only contract.

If I go via upgrade my plan I can view the SIM only option I want, add it to basket but then at checkout I get the message:

“These details can't be edited at this stage as we need to use the information currently held on your account for eligibility purposes. Log in to the iD Mobile app or My Account online and head to ‘Edit my Profile’ to change your details.” I’ve done these even though all the details were laready correct. But the same things happens again.

Does anyone know how to get this done? I don’t know if the SIM only option doesn’t appear because I’m still in contract, but I’d like to organise a seamless transition - rather than change after the contract ends and pay another month at the high rate. Many thanks if you can help.