If you are referring only to Eastern Orthodox Churches in full communion with each other then my understanding is that you can attend anyone of them. Usually, you would only change because you moved somewhere where the only Orthodox church was a different kind to the one you used to attend, or some similar reason.

​Of course, Oriental Orthodox Churches and the Church of the East are a different matter.

Not confirmation. The priest might want to make sure you were a properly baptised member of a valid Eastern Orthodox Church, but apart from that you can take communion in any such church. ​

Some Eastern Orthodox parishes in Western countries can be quite ethnically based, and not entirely welcoming to outsiders, so that might be a practical difficulty, though the priest will no doubt not put you off taking part in worship there. Other parishes, whether ethnically based or not, can be very open to outsiders.