I needed to do this because I imported a 3D model as a small ship, then refined it and decided it needed to be a large ship. But using SEToolbox was not a desirable solution because it "messed with" the ship during the export to OBJ and re-import of the OBJ. I wanted an exact replica but in large format and 50,000 blocks was just too much to re-create from scratch. This is the method I used to convert a small ship design into a large ship.

I could easily see how this process could be automated with a tool to strip all non-convertable parts off and resize a ship in either direction. But alas, I'm not gonna be the one to create a tool that does that. :)

Your world needs to be in creative mode if you want to enable Copy/Paste feature to backup your original ship. Or do "Save As" to make a backup copy of your world.

I would recommend that you use an empty world with only your ship in it to make it quick-n-easy to convert. Otherwise you will need to tip-toe around all the other objects in the savegame when you edit it.

Remove / delete all components that are not armor blocks. Some components do not have large ship equivalents and those that do are not the same size/shape (antenna for example).

You can use a cockpit to verify all components have been removed and then remove the cockpit last.

Find your savegame folder for the world you were just editing.

Example:

Edit SANDBOX_0_0_0_.sbs with a text editor like Notepad and turn off word-wrap. It is important that your editor does not change the format and spacing of the file.

If you did this with an empty world with only 1 ship in it, you don't need to avoid touching ships/stations that you don't mean to touch.

Find the following:
change it to:

Do a search and replace all for:
change it to:

Save your changes and close the file.

Start Space Engineers and load the savegame you were editing.

If it worked, your small ship is now a large ship. Now you can begin adding components back onto it.