Upgrading is COOL! Here's how I do it.
Hit F3 to see the ship designs screen. Select the ship design you want to upgrade. Hit the "Upgrade" button, and it will create a newer Version with the newest tech you have researched. Give it a new name and save the design by clicking the "Create" button.
Now, move a ship of the old design to a shipyard, select it, and press the "G" key. Now you have to select the ship again (dunno why) and click the "Retrofit" button on the right. You will then be presented with a list of the designs that the old ship can be retrofitted to. Choose the one you want.
Now, when you look at your ship, it will be all damaged, and you will say "Possum, you arsebite!"
Hehe, that's how it works. Just leave the ship sit there, and the space yard will repair the damaged components. After the repairs are complete, you will have your newly upgraded ship, but it will still have any experience it might have accumulated.
Not only will you keep your experienced ships this way, but it is usually faster than building new ships, not to mention cheaper. To speed up the process, I will usually research Repair tech, and build a repair ship to hang out at each of my busy space yards.
I am a big user of specialty support vessels. I will build repair ships, supply ships, satellite tenders, minelayers, and various other odd ducks, but the repair ships are always a priority for me.
Oh yes, and if you're building Weapons Platforms, don't forget that even the small ones can use a large weapon mount for their direct fire weapons. Click the "Weapon Mount" button on the right of the design screen to select
gods, but I love this game