While o.m.'s anwer is fine and gives you many of the German terms that may be relevant to you in the future, let me put another spin on this:
No, your employer cannot do a one-sided contract change without you agreeing. Your salary is part of your contract. It is aways possible to say "no" to any contract change your employer asks you to accept, without giving reasons.
Secondarily, Germany has excellent employee protection against being let go. As a general rule of thumb, as long as the company itself is not in trouble (e.g., approaching insolvency), and as long as there are no "reasons in the person or behaviour of the employee", it is very hard for the company to make someone redundant. (Note: all of this is true for companies of at least 10 employees, which I assume is the case for yours.)
So there you go. The most important advice for you is to not let yourself be pressured into signing anything. When your boss comes to you and asks you to sign, simply say no. And yes, I do know that this is everything but simple, but in this very specific case, it is the answer. Do not feel pressured into giving a reason, do not discuss this with your boss.
It is impossible for you to tell whether and how bad any repercussions will be, i.e. whether your company will try to mess with you later. They have no legal handle (unless Insolvenz looms, in which case you want to get out ASAP anyways). They may try to spread Fear, Uncertainty and Doubt, but this should not let you be pressured into signing anything.
The ball is firmly in the court of the company to give you a good reason to go along with their plans. As you do not give us any information about the situation, it is hard to give further concrete advice. Obviously it is always prudent to have a polished CV and at the very least put out feelers for potential new employers; the better prepared you are, the easier it is for you to stay relaxed in this situation. If push comes to shove, you find the names of all the relevant parties in o.m.'s answer; and if your company escalates, feel free to ask again.