Merger of top-heavy and non top-heavy plan

Two calendar year plans with unrelated employers, Company A and Company B with corresponding 401(k) plans, Plan A and Plan B.

Plan A will be top-heavy for 2018. Plan B will not be top-heavy in 2018.

Suppose Company A sells its stock to Company B on January 1, 2018. All participants in Plan A are merged into Plan B on January 1, 2018.

Thus, Plan A no longer exists on its after January 1, 2018.

Will Plan B have to re-determine its TH status for 2018 by including the 12/31/2017 data from Plan A? Do the Plan A participants get any top-heavy minimums for 2018? Or how is this handled?

Look up the definition of a Required Aggregation Group.  I think you will need to have plan B re-determine its TH status for 2018.

Agree with Mike. Once the plans merge, they are one and the surviving plan inherits all of the attributes of each constituent.