If you switch from single coverage to family coverage, you are allowed to contribute the family maximum for the entire year. If you switch from family coverage to single coverage, you are required to prorate your contributions based on the number of months under each type of coverage.
Example: Jane has family coverage 1/1/15 – 3/31/15. Jane switches to single coverage 4/1/15 and remains covered through 12/31/15. Jane is eligible to contribute $4,237 for 2015.
$6,750 ÷ 12 × 3 (for the family coverage time frame) plus $3,400 ÷ 12 × 9 (for the single coverage time frame)