Multiple Employer to Single Employer SH

[Benefits to all]

Looking to see if anyone knows of any IRS guidance out there that could help me out with the following.

Company A & B cover their employees in a multple employer safe harbor 401(k) plan that has fulfilled all proper notice requirements. Halfway through the year (effective 5/1), however, Company B is going to shift its employees into a single employer SH 401(k) plan by amending the plan to simply say it is no longer a multiple employer play, it is a single employer plan, and establishing a separate trust. It will have all the same features as previously in place, contributions, protected benefits, etc.

I think the argument is that it is the same plan, not the creation of a new one. It is a simple conversion; not the creation of a new plan. I cannot seem to find any IRS guidance on the issue. Any input/opinion would be appreciated.