Just to clarify, in terms of design it doesn't matter when you leave a league, the important factor is when you join another one. You should aim to join your new league during the inter-season break. If you do this, assuming the league you are joining is not in its very first season, your design will be pretty competitive, it will be as if you had an entire 17 races with your active CD.
If the league you are joining is a brand new league your design will reset to a very low level, but this isn't a problem because all of your competitors will suffer the same fate.
For a full explanation from the developers see the first post in this announcement...
New Design Balancing System
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