Possible Spinoff or Successor Plan?

We have a Safe Harbor plan that has two participating employers due to controlled group rules.  The employer that is the Plan Sponsor will soon have 100% of the company stock bought by a separate individual.  The new owner will continue as a participating employer in the plan.  I understand that now that they are no longer a controlled group, they will need to be tested separately, which should be no issue since it is a Safe Harbor Plan. 

My question - what if the new employer decides he no longer wants to participate in the current Plan?  Can he just "spinoff" the current plan as long as he keeps all the provisions of the original plan intact?  Or, if at some point in the future, he decides he doesn't want to maintain the Safe Harbor plan, can he terminate the participation of his company in the plan and start a new plan?  I assume this would have to be done at the beginning of a plan year, since it was done after the sale of the stock?  Is there anything I'm missing?  

If I understand you correctly (i.e., A owns companies X and Y and now B will own Y, and A and B are not related), then yes, the plan can either continue as a multiple employer plan (see sec. 413(c) and regs thereunder), which for a variety of reasons most employers don't want to do, or X could cause the existing plan to spin off the part of itself attributable to the Y employees, or vice versa, and following that the employers could amend or terminate their separate plans. For a termination you would need to navigate the 1.401(k)-1(d)(4) rules, but that should be doable.