How difficult is it to change the oil pump in a 1996 Seville with 4.6 Northstar on 1996 Cadillac Seville SLS

the computer flashed 'check engine oil', as I have been unemployed, it had been a long time since last change (about 20k). I changed the oil & filter. The same message now comes up and oil level is not low, although about a quart over now... Today the oil light (idiot light) flickered while I was at a stop light; engne was running to the hot side (but not overheated). This car does have an oil cooler also. How "probable" is it that oil pump is bad?

Asked by for the 1996 Cadillac Seville SLS