The common law covenants of "quiet enjoyment" and exclusive possession never excluded access in a bona fide emergency, and a landlord would be entitled, and perhaps required, to break a door or window open if needed to gain access in the face of an emergency such as a gas leak, where others are potentially endangered and time is of the essence.
That said, a valid and reasonable contract may specify this as the parties please, unless specific law overrides this. For example a local landlord/tenant law could require that the landlord be given access (such as a key) Aside from such laws, the lease can forbid changing locks, permit it only with permission, or permit it freely (perhaps requiring that the landlord be given a key). Any of those provisions would be lawful in the absence of a specific law to the contrary. The lease could also specify when the landlord is to have access, and how much notice. But a provision denying access in a true emergency is probably not valid.