Dual citizenship under Tier 2 and other complications

Hi,

My Tier 2 biometric card is under my Hong Kong SAR passport. However, it's about to expire and I have dual nationality as a New Zealand citizen. If I'm entering the UK, do I need to have a valid HK passport while entering the country? Or can I just show my expired HK passport with all the corresponding details alongside my New Zealand passport?

Also, I'm about to apply for an unmarried partner visa in January 2014. Will I be able to change my nationality to New Zealand on that visa application?

Ideally I don't wish to renew my Hong Kong passport. I've read that if I want to change nationalities I will need to do another application with more application fees and I want to keep all of these administrative processes to a minimal.

Please advise!

Thanks!