How are points re-calculated when changing leagues?
When you are promoted leagues, your points get reset. If you had 500 points in bronze, when you are promoted to silver, you might now have 400 points in silver.
How is this new score calculated?
According to TeamLiquid, your points in the new league are:
Rating Change
Consistently, we are seeing that people promoted into Master league are
assigned new ratings equal to 73 + spent bonus pool.
EDIT: This was confirmed to occur for all leagues, not just Master.
It was simply more evident due to the mass promotions that occurred
during the first day of Master league.