fuel injector seals

I need to change an injector in a 99 Grand voyager flex fuel 3.3. When
I change one injector will I need to replace the O-rings on the entire set?

I purchased this van a while back, and it has been sitting for a couple
of months while I figure out how to get it back into shape. After I
bought it I discovered that it had belonged to a friend, until he sold
it last year. He had always changed the oil every 3,000 miles, but his
buyer put 15,000 miles on it without changing the oil, and now that it
has sat for a couple of months one of the lifters has started clicking.
I have heard that you can sometimes get this resolved simply by
changing the oil a couple of times. Would it be advisable to use a
bottle of engine oil flush treatment? I don't think I have much of a
risk of gumming everything up because there shouldn't be that much crud
in there, considering the prior history of the vehicle.