IRC Sec. 432 says that the actuary's projections shall be based on reasonable assumptions and methods and be his best estimate. However, Sec. 432 goes on to say that the projected value of liabilities shall be determined based on the most recent actuarial valuation. What if the actuary's best estimate has changed between the time the actuarial valuation was completed (let's use a 2008 calendar year to make it specific) and the deadline for the 2009 certification (March 31, 2009)? Can an assumption used for the 2008 valuation be changed for the 2009 certification?

I don't see any problem with that as long as you use the same assumptions for the 2009 valuation.

Also, remember that the "valuation" isn't really official until you file the Schedule B. Therefore, although you may have released the 2008 valuation report, your assumptions aren't really locked in until you file the 2008 Schedule MB, which will most likely be after you do the 2009 certification.