Another important thing to note is that you can't make "lateral" equipment bids, i.e. no ERJ-->CRJ and no ATR-->Saab and visa versa, but you can bid "up" i.e. Prop to Jet.
When you upgrade, hehe, you can go into any equipment that your senority will hold. In fact, most junior captain upgrades go into the Saab to start (least pay).