Let me explain

I was hired onto a company during a massive in-sourcing of the IT department (Couple hundred jobs opened up) and was working as a contractor for a few months but was hired on as a Salaried employee. It states quite clearly in my offer letter which was signed and dated:
"Your base compensation in this position will be
at an annual rate of $xx,xxx. This will be paid on the 15th and the last business data of each month."
Later on down the road when they began hiring all the other people on my team they decided that we should be hourly instead and they all were given hourly offer letters and that's how we are all paid now, I did not get any additional paperwork or agreements. I also learned that some of my coworkers had negotiated sign on bonuses which I missed out on and is my fault for not thinking to ask.
Sadly, this is not the end of my tale.. They then decided that we were going to be paid weekly! Which is also outside the scope of my offer letter, and they promised to deliver a free full weeks check to alleviate the pain of this transition. They did, BUT, they shorted the checked just before the "Free check". I was mad, and asked payroll what the deal was and they answered that they had been prorating all those check before that to be +6 hours and they will not be doing that any more now that we are paid weekly. So some months I get paid what I was making but now I am making $800 less on any given month.
Since they pulled all this crap after my offer was set in writing (I got hired on way before my co-workers and before they made these policies) Should I try to slam them for a full years salary since that was in my offer? I am getting sick of this stuff they are pulling and as soon as I have my CCNP I don't think I will put up with it. Great company to learn, and grow, and I love my team/management but money has got to come first at this point.
It also hurts to know that the actual salaried engineers above us don't have to submit time sheets weekly, and are paid Bi-weekly like how I am suppose to be! The only would be advantage should be that we are now eligible for overtime but since we get paid separately for on-call rotation we have not had a need for anyone other than the on-call to get OT.
Thanks Guys,
Sorry for the rant!!!