Postdoc contracts in US - signing an initial one which is superseded by different online one?
From the OP's comment:


  When questioned, I was told that this is what the contract is now. 


Hmm.  Most of the point of a contract is that it cannot be unilaterally changed during the period it applies to.  So you can absolutely insist on the old contract -- in fact, just by not signing the new one -- but the last six words of the previous sentence are key.  If you signed a one year contract but have the expectation (or even informal agreement) of longer employment, then refusing to sign the new contract could place your future employment in serious jeopardy.

You are really not being treated properly, but you have to decide whether it's worth it to take action.  I couldn't be completely sure, but it sounds like you are saying that you signed a contract for a certain salary and are now being asked to sign a contract to cover the same employment period for a lesser salary.  Isn't that kind of the gold standard of unacceptable (and of course, illegal) employer behavior?  If I were in that situation I would not be willing to sign a contract lowering my own salary.  I would do my best to work the situation out amicably, but I would understand that this might lead to a parting of ways between me and the university.  

In case it's not obvious, this is definitely something to discuss with both your supervisor and the department head.  Their attitude might be "We know it sucks, and there's nothing we can do about it" but even so you should get their advice and, if possible, their support.