FWIW, this is from the ABA Employee Benefits Committee IRS Q&A in May 2009:
32. § 409A – Short-Term Deferral Exclusion If a payment satisfies the short-term deferral exception to Treasury Regulation Section 1.409A-1(b)(4), may the parties subsequently adjust the amount and timing of payments?
Proposed Response: Yes. If the short-term deferral exception applies to the original arrangement, then such arrangement is excluded from Section 409A. The original arrangement may therefore be modified provided that the modified arrangement also qualifies for exception to Section 409A under the short-term deferral rule.
IRS Response: Yes, but other tax doctrines such as constructive receipt continue to apply.