Replacing the old house with a new residential unit would, in itself, be unlikely to be regarded as a 'material change of use' (although there's no actual legal definition of the phrase). So that's one hurdle which doesn't need to be cleared.
However you will have to seek a formal decision as to whether the planning authority views the demolition (and subsequent redevelopment of the site) as requiring formal planning permission:
https://gov.wales/planning-permission-demolition