As with subjects any data which have been established can be deleted, and any changes in already established data can be signalled using は:

The information about "パーティに" and "いく" are unchanged, therefore they are deleted. The subject changed to the original questioner, so the change is signaled with は. The information about "パーティに" is still deleted, but "いく" comes back as "いかない" because it has changed form. The established information "パーティ" changed to "映画" so we use は to establish the change in understood information. Understood info that hasn't changed is still left out, like the subject "僕" and the action "いく".

This is pretty much how all anaphoric deletions work in Japanese whether it be subjects, objects, or something else. In general, always attempt to delete if possible. This section should also give you a feel for the use of は in normal speech.