While the previous attorney answer is correct generally, your question presents an additional complication...the fact that you have been employed for some time prior to this agreement being presented. Generally, if your employer where to say "sign this or we will fire you" and they don't offer you additional compensation for signing the agreement then, because you have worked for them and this was not a part of your original work agreement, they can't "legally" fire you. By "legally" I mean that if you were fired you would have a potential action against them. That said, they would likely still fire you, and you would be unemployed. You would have to sue them and seek monetary damages. And none of that is guaranteed.
All of this depends, of course, on your initial employment agreement/arrangement/contract. It is also depends upon Texas law (assuming you're employed in Texas) and I am not licensed in Texas. You should consult with a Texas Employment Law Attorney for specific advice on your issue.
Best of luck.