Starting 401(k), SIMPLE notice not given

Hello,

First post, hope this is the right place - I have a plan sponsor that wants to start their 401(k) Jan 1, 2021. However, they have had a SIMPLE, and they did not give proper term notice to the employees. Say they just stop contributing to the SIMPLE, as of the end of the year, and start contributing to the 401(k) - I know this isn't allowed , but what are the repercussions? If the IRS doesn't require to be informed of the termination, how would they even know? It seems like their violation would be not filing proper SIMPLE notices - but the 401(k) would still be valid, yes?

Thanks in advance,