To get the promotion you must be on the $55 plan as of Feb 29th, 2012.
To keep receiving 1 GB you need to stay on $55 plan all year. If your switch to a different plan and switch back to The 55 you will not get the promotion. If your account doesn't have enough money to renew you have a 30 day grace period to add money and renew the plan. If you miss that 30 day window you will lose the 1 GB promotion.