Yes you can replace just the whites first. IMO it's better to swap-in new bulbs in stages so there is less of a shock to the corals.
A split of ATI Blue Plus and ATI Aquablue Special would give you a crisp white/pale blue look overall. They've also got rediculously good PAR. If you want some more "pop" then you could exchange an AB Special for something like an ATI Procolor, UVL 75/25, or UVL Aquasun, or the swap a Blue Plus for a pure actinic.
Best place to order them is reefgeek.
-Justin