Replies are sent on writing paper with the address, as for official functions. They are traditionally addressed to the hostess even when the invitation is a joint one from both the host and hostess.
When invitations are extended to unnamed guests such as 'and Partner' or 'and Family', the reply should contain the names of those who will attend.
A named invitee should be substituted with another guest only if the hostess gives her express permission.
If an invitation is accepted, it is bad form to withdraw that acceptance unless there is a genuine reason (eg illness); another, more appealing, engagement is not an excuse for withdrawing an acceptance.