[neurotech1]

Missing the key point. Aircraft are updated with newer avionics after leaving the factory quite routinely.

Considering Southwest currently has 122 737-300s in service, they wouldn't upgrade them if it wasn't available at a reasonable cost:

http://www.airfleets.net/flottecie/Southwest%20Airlines.htm

Cockpit upgrades for 737s:

http://www.aviationtoday.com/av/issue/feature/Product-Focus-...

Cockpit upgrades for C172 type aircraft:

https://flyhpa.com/2012/08/the-garmin-g500-g600/