When UPS bought overnight you weren't able to just retire under the overnight formula start collecting your pension and keep working for UPS. the conditions of the pension as and how many years eligibility investment pretty much stayed the same. Up until 2013 it was the UPS retirement plan then it changed to the UPS pension plan, how you calculated the financial benefit changed but not how you achieved and were able to draw on your pension.
As I stated it's a good question.