Yes, and no. The EBA structure does work like that, and has done so ever since QF merged with TAA. But, all that means is that there are two different pay structures, and sets of rules. The pilots can, and do, change from one to the other other. 737 FOs all come from the SO ranks on long haul, and after their time in the little jets will go back to larger ones. The cycle may then repeat when command slots come up. So, you'll have many 737 Captains who were 330 or 380 FOs, and the same for the FOs. Twenty years ago, integration of a new type was more complex, because you had legacy people (from before the merger and the 89 unpleasantness), but that should have all washed out by now.