In short, yes, you should rewrite your smart contracts from scratch. How much you need to rewrite, it's based on project logic. I can share couple things for help you to make a decisio.
Firstly, FunC is not the same as Solidiy.You can find more details about this language and examples in the documentation.
Secondly, and I would say this is a very important point. Ton has a different architecture than ETH like chains. You can read this article to understand more how Ton works with smartcontracts and why. blog post