As far as the Phaser Arrays vs turrets on older ships. I think it has to do with the way the weapon slots were built into the hull along with all the support wiring that comes with it. Think about it, ship mounted Phasers need Power, Electronic communications, internal access room / space for maintenance.
If your hull wasn't designed for it in the first place, having to shove it on would be a major refit. You could probably be in space dock for who knows how long.
Especially in war time, you do what is fast and efficient. You can't have a ship sitting there for months or weeks.
Get the latest turret versions of the same phasers, install it, call it done.
Upgrade your shield systems to the latest model, same with engines and computers. Call it done.
Look at our real world US examples.
USS Midway started at the end of WWII and was decommisioned in 1992.
Excelsior and Miranda classes getting upgrades lasted over 90 years.
Any ship that is well designed and maintained can probably last several centuries with technological upgrades backing it.