I did exactly the same thing, but i started with a sidewinder, mined my way up to a hauler then got my cobra, found a good system and managed to get my self 1 million credits, so i thought i would upgrade things, i bought as higher class as i could, and when i went to the galaxy map to do a bit of exploring i found i had a jump distance of 1.8 light years. i gathered i had mucked something up so i went to change everything back and there wasnt a but back option (i didnt leave the station) the only way i could resolve it was to sell my cobra for a sidewinder then exchange it for another cobra with standard stuff on it, on the plus side i fitted it with a 6 bin refinery, as the station doesnt have the medium beam lasers or multi cannons that i had previously. i feel your pain, but it wont take long till you get back on your feet.
you learn by your mistakes, now i know how the class system works