Who is the letter from (not the individual's name of course, but in general terms was it the company, the trustees, the trade union etc?), and what was the basis for the writer's assertion that the pension may be affected? I very much doubt your father's pension will be affected in any meaningful way - they might just freeze it for a few years or change the way increases are awarded in future years.
If he is retired, he is already drawing down his pension. This means he will be among the last to be affected if there are changes to the scheme. One thing's for sure, his current pension will almost certainly not be cut. That is almost guaranteed. Scheme members who are currently working in An Post are the ones who will be affected the most. Pensioners like your father are usually spared.
An Posts's pension scheme is hugely in deficit, and it is in negotiations with its unions about how to fix that. So I'm not surprised a letter has gone out.
Are you sure it wasn't just a general letter sent out to all members of the scheme, to say that the scheme will change? Read it closely, it may say something about how already-retired members will not be affected.
Your father shouldn't worry too much. His pension is highly unlikely to be cut. Like I said, the worst that might happen is they'll change the way increases are awarded in future years. His former trade union should be able to tell him more.
(I know a thing or two about the An Post scheme, so private message me if you like)