There have never been any 757s produced with a single overwing exit on each side, the emergency exits are located behind the wing on aircraft that do not have overwing exits.
There always appears to be some confusion about the exits on the 757, and why there are 2 layouts.
To address the initial question, whether airlines choose to have their B757-200s with 6 Doors/2 Emergency Exits or 6 Doors/4 Overwing Exits is entirely down to their own preferences. There are no regulatory requirements, ETOPS or otherwise, that require a particular exit layout for a particular type of mission.
The only additional rules for ETOPS flights regarding exits are the additional liferafts required on the 757, which are stored in the roof or in overhead lockers adjacent to an exit - but the either exit layout is acceptable for ETOPS flights.
In fact the 757 without overwing exits is preferable for ETOPS missions because the liferaft can be placed out of the door directly into the water, and the slides from emergency exits 3L and 3R can be used as rafts. With overwing exits the liferafts have to be dragged over the wing into the water and the overwing slides don't detatch, so they cant be used as rafts!
The exit layout is also not an indication of an aircraft's age. Both configurations have been available since the start of the B757 programme...
All Delta's B757s have had 2 overwing exits from the first deliveries in 1985 to today.
All British Airways' B757s have a single emergency exit behind each wing from the first deliveries in 1982 to the last ones in 1997.
For reasons of commonality once a 757 operator has chosen an exit layout, whichever they prefer, there is very little chance of them changing that layout. The fact that Northwest did is an anomaly, and too much should not be read into their decision - I am unaware of the reasoning behind it.
As a quick summary, here are current operators by exit layout;
6 Doors/ 2 Emergency Exits
Air 2000 (ETOPS)
Air Holland (ETOPS)
Air Transat (ETOPS)
Britannia Airways (ETOPS)
Britannia Airways AB (ETOPS)
British Airways (ETOPS)
British World Airlines
Canada 3000 Airlines (ETOPS)
China Southern Airlines
China Southwest Airlines
Guyana Airways 2000
LTU International Airways (ETOPS)
Monarch Airlines (ETOPS)
North American Airlines (ETOPS)
Orient Eagle Airways
Royal Air Maroc
Royal Brunei Airlines (ETOPS)
South Atlantic Airways
TACV Cape Verde Airlines (ETOPS)
Trans World Airlines – TWA (ETOPS)
Uzbekistan Airways (ETOPS)
6 Doors/4 Overwing Exits
American Airlines (ETOPS)
Continental Airlines (ETOPS)
EL AL Israel Airlines (ETOPS)
Far Eastern Air Transport
Royal Nepal Airlines
United Airlines (ETOPS)
As you can see, your assertion that airlines have changed from one layout to another to meet regulatory rules is incorrect.
Only Northwest, National, Mexicana and Icelandair operate both versions, and the latter 3 only through second hand aircraft acquisitions. And far more airlines operate ETOPS flights without overwing exits than with.
Hope that helped clarify things slightly...I'm open to any corrections on the above list and have excluded Corporate and Military operators of the 757.