Hi, all --
There is no clear guidance from the IRS or Treasury about how to determine HCEs in the year of a stock acquisition or a business merger. If you look at the Code and regulations, the clear intent of the rules was that there would be one applicable determination of who is an HCE and then that would apply across all plans. But, in a stock acquisition, particularly where the acquired company and the buyer each sponsors its own plan and the plans will operate separately during the transition period, it is not clear at all how to determine HCEs. So, I think it makes the most sense, and is defensible as a reasonable interpretation of the law, that you maintain the pre-existing HCEs from before the acquisition vis-a-vis each plan for the year of the acquisition.
A couple of additional notes: first, the transition rules take you out of coverage testing, which relieves you of needing to define HCEs for that purpose. But, the transition rule does not relieve you of nondiscrimination testing. If your two 401(k) plans are just that, then you can go ahead and test them separately and the use of the prior HCEs in the year of transition probably makes the nondiscrimination testing harder to pass than if you had some kind of cross-company definition of HCE (i.e., there is some possibilty that people who were HCEs in the acquired company would become NHCEs due to the top 20% rule or something similar). HOWEVER, remember that, if any of the plans use cross-testing and you use the average benefit percentage test as part of the cross-testing, then you need to take into account benefits of all plans of the company ... which means that the definition of HCE becomes problematic from that standpoint. So, you may need to look at this a little differently in that circumstance.
Last but not least, look for situations in which any of the assumptions about what the rules might be if the IRS/Treasury actually wrote them creates a skewed result which is abusive in nature. So, let's say that you make a reasonable assumption about who the HCEs are, and it turns out that, with that reasonable assumption, the amount that the HCEs get or can contribute quadruples from prior years. Just be careful that you are not creating a situation where the IRS would be tempted to exercise its rights under the coverage and nondiscrimination rules to consider something abusive and plan-disqualifying.
Hope this helps.
Everyone stay healthy!
Ilene