A client of mine is interested in updating its Plan's definition of Actuarial Equivalence to a more recent table. Are there any issues in doing that? I believe we need to preserve the old basis (as a minimum) during the year of the switch?

Where do you see this?

If you are just updating the plan's definition of actuarial equivalence (rather than the special 417(e) assumptions), I believe that you just need to protect (grandfather) the PVAB as of the later of the effective date of the amendment or the date the amendment was signed.

David,

Are you questioning the need to protect the value of the optional forms on the old basis (ie: 411(d)(6)) or are you questioning her statement that she only needs to do it for one year?

The change in the defn of actuarial equivalence table I am referring to applies to optional forms (non 417e types)

Consider whether a 204(h) notice is needed. See the following from 54.4980F-1 A-7(a)(1):

"All plan provisions that may affect the rate of future benefit accrual, early retirement benefits, or retirement-type subsidies of participants or alternate payees must be taken into account in determining whether an amendment is a section 204(h) amendment. . . . . Plan provisions that may affect early retirement benefits or retirement-type subsidies include . . . actuarial factors used in determining optional forms for distribution of retirement benefits."

The latter, assuming the discussion is not focused on changing the lookback month for a LS present value.
I think the response from carrots is correct, except that each optional form stands on its own for this comparison.

Does it mean we'll need to grandfather the individual optional forms at whatever age it is under the old basis going forward if it is indeed more favorable?

Depends on what you mean by your statement (not very helpful, huh?). But the general answer is "yes" insofar as the definition attaches to benefits that have already accrued. Accrued for this purpose means the satisfaction of all criteria to earn a benefit other than the mere passage of time, I think.