The Lisbon Treaty makes the € the currency of the EU members of the €zone. All new members have to join the €, but current members with opt-outs, such as the UK, do not need to join. No treaty change needed.
All Cameron is asking for is that EU members who are non-members of the € are not treated differently, but that is not proposed anyway. If it did happen it would kill the EU/EEA and EFTA agreements, and both sides know it. It is a non-issue.