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That is likely a hybrid of Dendrobium nobile and related species. The time to repot is when it is making new growth. New growths form new roots, and need to be kept quite moist, else they wilt and don't grow to their potential. That is happening now.
The growths I see and aerial roots are coming from new plants forming on the upper parts of the stems. These extra plants are called keikis. I can't tell whether the base of the original stem is still alive.
I would very gently use a razor blade to separate the plantlets from the stem, then pot them up separately into a pot that just holds the roots mass. For potting medium you could use fine or medium orchid bark. Water the plant well during its growing season, so it never goes dry. During the winter it will be fine with watering every 2 weeks.
You can read more about this kind of plant here, at the now gone but archived version of the Yamamoto Dendrobiums US Web page:
Yamamoto Dendrobiums: Caring
Yamamoto Dendrobiums: Hints