This is getting more complicated than I anticipated and I think points other than my original question may have been introduced. For the sake of simplicity and trying to bring this to closure, what I am saying is that the Code and regs are clear that you are TH or not for "a year" (say, Y1) based on whether more than 60% of value as of end of prior year (say, Y0) belongs to keys. The regs are clear that you also determine who are your keys for "the year" (i.e., again, for Y1), based on participant attributes during the prior year (i.e., during Y0). The Code is ambiguous on the latter point, but the regs and, as it turns out, legislative history and practicality, trump the Code's ambiguity on that point.