You can use this simple calculator to understand if there's value in converting a Help to Buy ISA into a Lifetime ISA.
Help to Buy ISAs and Lifetime ISAs have different rules associated with them. Whilst it is possible to have both types of account, when buying a house, only one can be used to claim the government bonus. There are also penalties associated with withdrawing money from the account when it is not used toward buying a house.
The Help to Buy scheme will provide a bonus from the government of 25% up to a maximum bonus of £3,000 and can be used toward the purchase of a house of up to £250,000 outside of London and £450,000 within London.
The Lifetime ISA scheme has a maximum yearly limit of £4,000 and a government bonus of 25%, and can be used to purchase a house of up to £450,000.