You can leave the leaves on until they lose all their green. These plants lose their leaves from the bottom up, faster in the summer when they are growing new ones, too. So yes, there is a slightly yellow one on the bottom, and the second one does have a brown spot but it doesn't look like anything serious.
Phals also hate to be re-potted, and also can object to being moved around, so find a good spot for it with some early morning sunlight at most, and leave it there until it's finished blooming. Then think about re-potting if necessary. Btw, it's a VERY good thing to see fat, healthy roots on top of the medium like that. When Phals are growing well they stick their roots up into the air, so don't feel as if you need to cover all the roots. They're epiphytes so they take in a lot of moisture and nutrients through the leaves as well.
Another very good thing is that the plant is sitting on a slant in the pot. This prevents any water from sitting in the cup of the leaves which can cause crown rot. So when you re-pot it, remember to keep it at that angle, too.
