What changes after peer review but prior to final submission are acceptable for a conference paper?
Following the acceptance of a paper, I would not make any changes that go beyond "editorial"—that is, improving the grammar, or adding a recently published citation. These do not change the "technical" content of the paper.

Anything where you make edits that change the actual research or results presented in the paper should be presented to the editors of the journal or conference in question, with a request for guidance. The individuals in charge can then make a determination whether or not additional peer review is required.