Maximum monthly guarantee tables (PBGC.gov)
Your pension is safe up to the maximums listed, if your pension is more than the maximum it will be cut to the maximum amount if PBGC takes over the plan. Early retirement (say you retired at 60 but are now 65) will cause the amount to be reduced.
As for underfunding, it is my understanding that the funding status considering the company is a going concern, I believe GM would be woefully underfunded if the plan were to be terminated. The last estimate I saw was as of 2005 where GM claimed 2 billion overfunded as a going concern but the calculation showed 31 billion underfunding if the plan were terminated.
Is GM pension fund $31 billion short? - International Herald Tribune
With the recent stock market decline underfunding is quite likely.
I am about to go through this with a different company. Here's what I believe to be correct:
1. In order for PBGC to take over a pension it insures, the pension must
be underfunded, and Co. must be in Bankruptcy.
2. Each Pension is then administered according to the language in that
co.'s plan document. so details may be different for each case.
3. Employees are divided into different groups depending on plan
language concerning eligibility for retirement.
4. Top group are those employees who have been
eligible for retirement
for 3 years. they would have their pensions payments reduced by a
factor computed by assuming they retired 3 years before plan
termination date.(whatever penalties plan doc. specifies)
5. Other employees who don't meet the criteria in 4. above would
probably receive
no more than the amount from the PBGC maximum
table cited.
If a person is 3 years older than
normal retirement age specified
in the plan they probably would see minimal impact.
This gets really complicated. You can read some of this at the PBGC website.