Q about weapon platforms

There's no retrofitting of units in SE5 at this time.

Space Empires Depot | SE:V Balance Mod

Build the new ones, then scrap the old ones.

The upgrade construction queues is not very intuitive. After designing your new unit make sure that you have the old unit selected then hit the upgrade queues button. It should present a dialog to pick the new design to upgrade to. Of course you have to have old units waiting to be built for this to work. It doesn't upgrade units that are in the process of being constructed. As the others have pointed out units that have already been built can't be upgraded.

Honestly I'd not scrap platforms unless I needed to free space on the planet.

Well.. If you have hundreds, far away from the front... Then there can be a significant maintenance benifit in scrapping piles of old models and replacing them with some 1 hightech one. Should save some cash.
~Myrath

OR you could ship them off to other planets.

True. That's useful for newly colonized worlds near the front. But the old weapon platforms are generally still 'junk'. Frontier worlds are better equipped with newer platforms.
But I can see your point that it may be interesting to ship them around...

~Myrath

Well, think of it this way. Junk is still better than nothing.

I personally like to move WPs around with my troop transports, instant defenses whenever you just invaded or colonized a planet.. dont have to wait for them to be built on site. It does require many ships to move them around since they use up lots of cargo space. With later techs and some weapon mounts WPs are really deadly support fire Imho.
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Without ammo, nice guns are just sophisticated clubs.