Migrate from Archicad 26 and earlier
MEP Modeler (now MEP Designer) was entirely renewed in terms of interface, modeling and technology, beginning with Archicad 27. It is not compatible with any older-version Archicad projects (26 or earlier) containing MEP routes and elements.
When you open an older-version project, you will see the MEP elements as ordinary GDL objects.
You can attach new MEP routes to these GDL elements. If you back-save to Archicad 26, your original MEP model is unchanged, but any new MEP elements added in Archicad 27 or later will disappear.