Also, your idea of "easily" is amusing to me. It's not trivial to just change from a trunk to a hatch, there is a lot that goes in to every change. You now have a new piece of glass to have someone manufacture, you now have to manage inventory for it. Same goes for the seal.
next is the hatch body panel. You need to make this in house, so you have to design it, create the die, etc. more inventory management.
done, right? Nope. Interior trim is now different and will require several new parts. Same stuff as above applies. The lift hardware, too. So on, so forth.
But wait, that's just the hatch. You also need to change the quarter panels, and their glass, with all the problems above.
Also the shelf behind the seats, and thus the seats themselves in all likelihood.
the roof, and thus roof glass, seals, and body panels for the upper section. Headliner. Probably the cross support.
possibly the rear doors.
the carpet, even.
so now you've made all these changes, which means the bill of materials for the model 3 has changed dramatically, and now all model 3 builds are affected because previous assumptions are no longer valid.
manufacturing isn't simple and changes are rarely easy.