For a quick try, if you have all three files ready for each of your new ships, you can also replace an existing ship model files (.cmp, .mat and .sur) with one of your new ones if you just put them in that ship’s folder and rename them to the same name as the ship you re replacing (but first, rename the original ship files to something else, for example old_li_elite.cmp etc), so you don’t need to change any files.
The icon in the shipdealer will show as the original one too unless you have made one for your ship.
But when you buy the new ship, remember to dismount all equipment and weapons first in case the hardpoints are not the same class or quantity, and mount new ones. If you don’t do this and the hardpoints are different between the ship you replace and the new ship, it can cause a crash.