What kind of dendrobium are we talking about? Do you have a tag or photos? That might affect the answer. Also, how do you intend to grow it? In a window? Greenhouse? What are your temperatures?
Generally speaking, dendrobiums need to be in small pots with a tight fit, so moving up a size might not be the best idea. Also generally speaking, if the plant is actively putting out new roots, you should be fine to repot. I do hesitate to repot things this time of year, though. For the most part, this late in the year, it's almost always going to be better to wait until at least until January or Feb (if not later) when the days are getting longer instead of approaching the shortest day of the year. Given that you just acquired the plant, however, it's also a good habit to repot sooner than later, so you can inspect the roots and better evaluate health. I guess overall it's a wash. I'd probably just leave it a couple months, but it's a coin toss, to me.
