Where is it specified that a plan must satisfy cross-testing requirements as of a certain day in the plan year (other than the methods available under Treas. Reg. 1.410(B)-8(a), for minimum coverage rules)?

Is L/D of plan year always acceptable?

And can cross-testing (new comparability) be performed based on a short plan year, including a short year of compensation?

Since you more than likely use the ABT for the general test, you must use the annual testing option, as prescribed under Reg 1.410(B)-8(a). Also, you must use the same testing option for 401(a)(4) testing, and since the ABT must use an annual method, I have always interpreted that end of the year data satisfies this requirement.

If your plan year changes and causes a "short plan" year to occur of less than 12 months, I don't see why you wouldn't be able to cross-test a new comp allocation. The only issue I see is making sure that, if necessary, you are limiting and prorating any and all 415, 401(a)(17) and 401(l) limit(assuming you may use imputed disparity).