Echoing the above, now is the time you want to build up your healing gear: not spell damage gear, unless it has more healing then your current piece, but actual +healing gear. You don't really want to respec when you don't have the gear to support your new build, particularly as a healer/tank, so waiting till 70 is probably going to be what you'll want to do.
I wouldn't get rid of your tank set though, unless you really need the money or know you won't ever switch back to tanking. That's one thing about classes who need completely different gear for each spec is you gotta be willing to carry "off-sets". I've never been full Holy, but for the most part, I've always had a healing kit, as up to date as possible without taking something over an actual healer.
One thing I'd suggest is try to avoid grouping with Paladin healers so that any plate healing that drops you can receive. For that matter, try to go with a Priest first, then Druid, then Shaman: that way you'll have the best chance to get your healing pieces as Priest can only take the cloth, Druid cloth/leather, etc.. Of course some circumstances won't allow this "bias", but when possible try this out ;)
Like how I try to avoid physical DPSers when going to BM on my Shaman to maximize the chance of getting that awesome trinket there, hehe ;)
Edited, Mar 25th 2008 1:52am by Maulgak