Solo 401K - the unknown "Now what?"

Mark and Mat for president 2024…just sayin, I’d vote for it.
I have a Solo 401K and have had one since 2017. I have approximately $300K in it. This year is the first year I own a business 100% with employees, which as I understand, now disqualifies me from contributing to a Solo 401K. I know I can create retirement options for my employees and myself i.e. safe harbor 401K, etc, but what about my old Solo 401k?? What happens to that, do I have to roll it over into something else? Can I continue to invest/manage that myself as I have been and just not contribute to it anymore, or is that off limits now? Thank you for your expertise. By the way, I did the $1200 tax attorney review and trifecta planning, but didn’t get around to this question. Worth the money for sure, to anyone else on the fence.