How to refit old ships on the workshop for A12.

Open the console and type:
di

This will show you (at the top middle of the screen) information about the ship and block under your targeting reticle.
Important are third and last line:
The ship ID (ID=1066 'The Ship Name')
The internal block name (382/HullThinSmall)

Now you can replace blocks like so:

replaceblocks <shipID> <oldBlockName> <newBlockName>

On most ships its sufficient to replace RCS blocks with empty space and armored blocks with stell blocks to get the ship working on tier 4.
The blocknames differ for HV, SV, CV and BA! Just look at the block you want to replace to get the correct name.

replace 1066 rcsblock empty
replace 1066 hullarmoredthinsmall hullthinsmall

After all that place some tier 2-, 3- and 4-cpu-extender and the ship should come to life.