Repotting in bloom will not kill the orchid. In many cases it will not kill the flowers (althogh they sometimes droop as a result).
What is more likely to kill the orchid is leaving it in medium that is no good for it.
To be honest I've never had any luck with Phals I bought in moss. I usually buy them in bark and the only 4 times I have ever bought them in moss they have died not long after I have bought them. (All my ones bought in bark are still alive and doing well).
Two I tried leaving in the moss, one I pulled out some of the moss so it was not as packed, one of them I completely repotted. All of them died.
But others here on OB have done fine with ones in moss, so I guess I've just been unlucky
I like growing in bark but the advice on what you should grow in depends on a lot of factors. Take a look at the advise from our member Ray as I think its good.
Chooseing Potting Media