Yes, landlords can switch from the current letting agent to lettingaproperty.com.
If you're in an initial fixed term with your agent, you must check your service terms to find out how long your term is, if you can end your contact, how much notice you must give and if there are any charges.
The initial term can vary from agent to agent, but it is usually 6 to 12 months.
If the initial term has ended, or you're approaching the end of the contract, you may be able to give notice in line with your contract. Giving notice to your agent does not have to coincide with the end of your current tenancy as it is the agreement with your agent that matters.