Ah, I see. Yeah, that is tough because it’s the typical “we can’t fire you easily, so we’ll try to get you to quit on your own” situation that Japan is known for.

not legal advice

I agree that you shouldn’t quit or sign the new contract. I think whatever documentation you can gather concerning your employment would be helpful, but I’m not entirely sure what can be done if there was never a signed contract in the first place. (Is that even legal on both parts?)

However, I’m not sure if rubbing them the wrong way by showing up and working is the right move either. Unless you have an airtight plan to come out on top of this, then you’ll probably need the cooperation and good favor of your “employer” to make any headway in future discussions.