Paying back relocation when orginial terms were changed by my company

I signed a relocation package in Sept 2011 for a new position which included a higher base salary and a 50% bonus potential, this was a signed offer letter. My relocation agreement states I must pay back the full amount if I leave within two years of the agreement.

In October 2012 my company unilaterally changed my bonus compensation from 50% to 30%. This was done across the board and with zero warning. I never signed an agreement stating that I accepted the new bonus structure it was just sent in an email.

I moved because of the pay and the potential bonus, relocated my family, short sold my house because I knew I could make it up. With the change in compensation from my orginial agreement am I still bound to the relocation agreement? I would have never agreed to move so far away and lose so much without the hope of future financial gain.