Hi Karl
The important factor is not when you leave your current league it is the point in the season that you join your next league. The ideal time to join is the inter-season break between Abu Dhabi and Australia. If you join at this point you will suffer almost no design penalty, your start of season design will be as if your active CD had been designing your car for an entire 17 race season. So if you have more than one CD make sure the best one is active when you join your new league.
If you join at any other point in the season you will suffer from a design penalty, the later in the season you join the more difficult it becomes to claw back the design. If you join an active league late in the season it will be very difficult to avoid relegation.
For full details read this announcement:
New Design Balancing system when changing leagues