I'm having a bit of trouble thinking this through:

Benefit Frozen 1/1/1993. The participant's frozen monthly AB at that point is $1000

Participant reaches NRA at 1/1/1996, but continues working.

Effective 01/01/1999, new formula applies to post 98 svc. The Benefit is the Frozen AB at 01/01/1993, plus the new benefit formula times service after 01/01/1999.

Lets say that the new formula is 2% times Post 98 service and the participant's salary is $100,000.

At 01/01/1999, the participant's benefit is the $1000 actuarially increased from 1/01/1996 to 01/01/1999. Correct? Lets say this is $1,200.

What is the benefit payable when the participan retires at 01/01/2009? Greater of one of the following?

a. $1000 actuarially increased from 01/01/1996 to 01/01/2009; or
b. Post 98 service benefit through 01/01/2009 ($1,666.67), plus $1200. Should the $1200 be actuarially increased from 01/01/1999 to 01/01/2009?; or
c. Post 98 service benefit through 01/01/2008 ($1,500.00), plus $1200 (actuarially increased from 01/01/1999 to 01/01/2008?), actuarially increased to 01/01/2009.
d. Something else?

Just my opinion, but....I say the greater of (a) or (d),
where (d)= accrued benefit at 1/1/09 = $1000 plus post-98 accrual.
Of course, the exact wording of the new benefit accrual formula is important, as well as plan wording of the actuarial increase provision.

Concur with Mr. R, provided (as he said) this is supported by the Plan document. However, if NRA was 65, I believe you would need to perform the calculation as if retirement at 70 1/2 (Mr. R's (a) + (d)) and compare actuarial increase from there with total benefit at 1/1/2009.

Thanks for the replies

You are describing 13 years in your example from NRA in 1996 to assumed retirement in 2009.
One of the document variations that is fairly common involves the ratcheting method.
You compute the actuarial equivalent of the benefit on the prior year anniversary, and compare to the plan formula at the current anniversary. In some benefit formulas, the new formula may be more valuable for the first few years of postponed retirement than the late retirement increase on the prior year.

This ratcheting method always assures that the participant gets the greater value.

Naturally, the process only applies if it was selected in the plan document.

Depending on the details of your plan, this assures that the answer from David Rigby is the minimum value you would provide, and might go even higher.