Your indefinite contract is practically untouchable by your employer's initiative without justified cause. The law protects you pretty well here: your boss can't just decide to convert you to temporary work because they feel like it. If they tried without a legitimate reason (like a serious company restructuring, economic, organizational or technical causes properly documented), we'd be talking about wrongful dismissal. And that gives you the right to claim damages, compensation and reinstatement if you request it.
What could happen is that the company goes through real economic difficulties and wants to negotiate changes to your working conditions, but that would require your agreement and can't be one-sided. If they pressure you without serious grounds, you have the right to refuse and keep your contract as is. I'd advise you to document those pressures (emails, conversations with witnesses) just in case things get ugly.
My recommendation is that you talk to an employment lawyer or get in touch with your union if there is one at your company. They'll tell you exactly what legal room your boss has according to your company's specific situation and they'll help you if you need to defend yourself. Don't let them intimidate you: the law is pretty much on your side with an indefinite contract.