Change from Grouping to Seriatim - Funding Method Change?

Believe it or not, I am working on valuing a DB plan that is only now moving to using individual data.  Back in the 80s/90s, when this change was happening how was the change defined in the FSA? (it's before my time) Any feedback in this would be helpful.

Wow.  As I recall, there was very little guidance (maybe none) in the 80s.  The first item I recall was Revenue Procedure 95-51 (I'll post a copy or link if I can find one), then Rev. Proc. 98-10 (https://www.irs.gov/pub/irs-drop/rp98-10.pdf), and then Rev. Proc. 99-45 (https://www.irs.gov/pub/irs-drop/rp-99-45.pdf).  

Much better consolidation and advice in Rev. Proc. 2000-40 (https://www.irs.gov/pub/irs-irbs/irb00-42.pdf).

RP95-51.pdf