The MEP Modeler for Archicad 27 has been entirely renewed in terms of interface, modeling and technology. It is not compatible with any older-version Archicad projects 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 will disappear.