I should also point out that there is some debate on exactly which date an administratively converted civil partnership to marriage will use. From my understanding, it should be the date you originally got your civil partnership and not the date of the changeover, but some have said this isn't the case.
Converting from a civil partnership to a gay marriage through a marriage ceremony will be dated on the day of the marriage ceremony. This isn't disputed.
Gays will still be able to get civil partnerships, but understand that with the UK (except Northern Ireland) having gay marriage, you probably won't be entitled to the same rights internationally as you would have with a marriage.