I don't like to repot anything unless it is absolutely necessary.
With that said, I have repotted phals in bloom with mixed results.
One. as someone mentioned, it can be tough as the flowers on a long arching spike can easily be ruined in the process.
Two, because the plant roots had been accustomed to constant moisture provided by the moss, sudden change into drier and open mix tend to upset the plant and buds are lost (but never all of them for me).
So I no longer repot but just wait until the moss dry out, then mist heavily (watering and waiting to dry can be risky but heavy misting (not the leaves but the top of mix until sufficiently moist but not soggy is the key I found!). I eventually repot into more open mix whenever I feel like.
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