Assumption of sponsorship to-do items

Say Subsidiary is dissolving but its benefits plans are moving to ParentCo. Aside from some definition and title changes in the plan docs and an assumption agreement signed by both companies, what else would go into a to-do list for ParentCo to assume the plans? Filing Form 5500 and 401(k) safe harbor notices as usual? Any notices to employees of plan sponsorship change? New to benefits work and feel like I'm missing something.

I'm not sure if you've provided enough information but it sound like you are merging SubPlan into ParentPlan with all that would entail.