Can I refuse to sign anything until I actually understand what's changing?

Yes. You are entitled to take the time you reasonably need to understand what you're actually being asked to agree to before you sign anything, and refusing to sign on the spot, without adequate explanation, is a reasonable position to take, not something that should cost you your job on its own. A demand that you sign immediately, without a real chance to review the terms or ask questions, is itself worth treating with suspicion — a legitimate change to your employment terms doesn't usually need to be rushed through before you've had a chance to actually read it.

That said, refusing to sign anything indefinitely, without ever engaging with what's being proposed, is a different posture from asking for reasonable time and clarity — the practical goal is getting a clear answer about what's changing, not stalling permanently.

Ask specifically what's different from your current terms, get it in writing, and take a reasonable amount of time to actually review it, ideally with someone helping you understand it, before deciding whether to agree. If you're pressured or threatened over simply wanting to understand a document before signing it, document that pressure too, since it can matter later if the situation escalates.