When calculating your cost basis, have you also included your reinvested distributions? Many people forget that, but perhaps your broker keeps track properly...
When you say that you are in the 15% bracket for cap gains, I'm assuming that means your overall federal tax bracket is 25% or higher, because if you are in the 15% tax bracket, then your federal long-term cap gains rate is 0%. Of course you still pay CA-taxes. And of course your sale might bump you up into a higher tax-bracket.
If it's a question between selling everything now and selling some now and the rest later this year, it doesn't matter for tax purposes. I would probably be inclined to sell now, because after the recent market decline, your cap gains will be less than a few months ago. Of course you'll buy the new fund at a cheaper cost basis, so you'll pay those taxes, whenever you sell that, but hopefully not for many years. And if I were inclined to gamble and believe that the market would be down more in 6 months, I'd sell half now and half in 6 months to lower my cap gains even more. But I'm not inclined to gamble and probably would just get it over with.
One other point to consider is how to pay those extra taxes: You can wait of course until April 2016 and pay taxes due on your 2015 return, but the IRS and/or the California Franchise Tax Board may assess a penalty, unless you fall under their "safe harbor" provisions. You can also pay quarterly estimated taxes, but - assuming that your are employed - the easiest way to avoid a large tax-due bill in 2016 would be to add extra withholding to your Federal W2 and California DE4 forms with your employer.
Only you know yourself and your situation best, so I can only give a few pointers for consideration, but can't advise you in any way.
