Hi, Bluebird! I am new, too, and only registered on the forum last week, but I've been reading and watching videos for months now. From what I have read, it's not necessary to dump your mix and start over, but it's actually more beneficial to reuse, as the dolomite works better after a year or so. Here is a video instructional for you (hope this works):
https://youtu.be/QpFM50ExS6k If this link doesn't work, go to the EarthBox home page, find videos, then look for the one that says Replanting Your Earthbox.
From what I've read, the mix can be used a decade or so. I think horticulturist Gary Bachman (who is a Global Moderator known as gardendoc here) has used the same mix for up to 13 years. Happy planting!