If that exchange is not scam and does not plan to scam customers, they will announce their KYC policy change and give their customers enough time to decide one of two options:
- Withdraw all money, refuse to complete KYC.
- Submit KYC, finish it and use full service of the exchange.
Not only exchanges but every platform that implements KYC and does not inform their existing users should be considered a scam platform. For example, OKX users didn't need to do the KYC, but they started implementing KYC. Kucoin, Bitget, and now OKX are asking for KYC from users, and they are doing it by publicly announcing this.
Sometimes, KYC does not guarantee that users' funds are protected if they keep them there. Coinbase started closing accounts even though users were KYC verified. I did not check what was going on.