..Again, curious to know how "physically based rendering" has been implemented in Lumion!
Lumion 5.0 uses a proprietary technique for the new materials.
The implementation is to establish the fundamentals of PBR in Lumion. So by necessity, it is not possible to provide an out-of-the-box complete solution straight away for all types of materials, nor for all variations in material types. Once the foundation is laid, then more materials and types can be provided as needs determine.
Most materials are defined, at least the basics of, in your 3D modelling application. You would continue to do that. When the model is imported, textures applied to surfaces in the modelling application will get exported/imported for use in Lumion. If there is a need to refine the look of the texture, for example, add depth to a brick texture, then you can add a normalmap for that texture. If the material imported is just a base (color or texture), and you want to make use of the (extended) Library of Lumion materials, then you can do that, as in the past.
The interface for users to edit the materials has changed, but not significantly in that there's a whole new way (e.g. the Max Slate Editor). You will be able to pick up and go with the new UI, it is reasonably familiar to you. Neither is it complex. The Lumion implementation of PBR itself is hidden in the background for you. You will see textures and sliders to change properties, stuff you have been familiar with in the past. Because Lumion is realtime, you get to see how the material will look in Build Mode
or in Build With Effects
You are given a new, more direct way of accessing any surface, that the developers hope, will improve editing times and freedom of access. It also provides more flexibility for new materials, as well as meet some feature requests from users.
Given the changes that PBR is compared to the current materials system and look produced, I would say that it will be best to complete v4 scenes and renders, and start new projects with v5. This is not an absolute, just a guide in terms of ensuring users avoid unneeded time trying to replicate exactly how things looked in v4. The time required to update materials to v5 may of course be worth it, and so some users will choose to move their scene to v5.
Some projects extend over Lumion versions, so it will require users to re-apply materials for v5. The developers have matched all current material types, and tried to match as many common features between the old and new, so that models can be imported with materials, or for currently worked on models. This has been done to ease the transition. In other words, Lumion will read in a 4.x model, but you will 'most likely' have to re-apply, or change the property settings for your materials in v5.0.
As Morten mentions:
... 'most likely' because the developers are still working on it, so everything I say here is subject to change until Nov 3.
What may take a little getting used to, is the change in look. Not for everything, but for some materials, overall for the scene, the way objects interact with light, and in some cases in subtle ways. Looks that improve the realism/quality of the scene.
Hope this answers your question.