Hourly Bonus pay dispute

I work as an RN in the healthcare field. Since RN's are in short supply, the network I am working for frequently offers a small hourly bonus past your required time commitment. I currently and commited to working one 8 hr shift (a .1 FTE) in a two week period and until recently as long as I worked my required commitment I was able to receive a bonus to work shifts where it was offered.
Our network has recently come forward to say that in order to receive the bonus shift you must have a work commitment of 32 hrs a pay period (a .4 FTE)and since I am classified as a .1 FTE then I will never receive compensation for the bonus shifts no matter how many hrs I work in a pay period be it the required .4FTE hours or full time hours. This has just come out after the next 6 weeks of open shifts was posted and bonus shift were signed up for by many employees that are just a .1 FTE. My question is this-Since employees commited to working the posted shifts with the understanding that they are bonus shift by signing there name on a list and then finding out only after they have commited to working the shift that they are not going to receive the bonus legal? Does the physical signing up of bonus shifts on a list form a type of contract between employee and employer? If the employer changes the rules after you have commited to the shift does this break any implied contract rules therefore not making you commited to working the bonus shift without bonus pay?