Hi,
I am not an expert, so my post is based just on my opinion an knowledge.
Category MTOM 600kg is based on aircraft design and technical solution as first place. National status and conditions are the second..
From this point of view if aircraft was designed/certified for MTOM 472,5kg already, there is no way to extend his MTOM up to 600kg.
If designer will create new calculations and design for MTOM 600kg then new aircraft can be approved for such conditions.
As an example, we have already national regulations for "special category aircraft - flying sport device" with MTOM 600kg but even though we have still fleet of older aircraft designed for MTOM 472,5kg approved for operations earlier than 600kg regulation was put into effect.

LSA is in Europe not a defined regulated category. In contrary to the US system where LSA is a separately category of aircraft that are subject to their specific Light Sport Rules.
In the EU it is different. There are National or EU rules for aviation. The split if one or the other is applicable is defined in the EU Basic Regulation. The latest change in the EU Basic Regulation has affected this split.
To focus on fixed wing aeroplanes. There is now a kind of range between 450 and 600kg MTOM, where these can be either in or out of the EU system. There are so-called OPT-IN and OPT-OUT provisions that give Member States the possibilities to either regulate this range of aircraft either at EU or National level.
Related to this range at the lower end of general aviation, EASA is working on changes to improve a more proportionate approach when risks are lower. There is a dedicated page on the EASA website that explains more details. This is anticipated to provide less burdensome options for the certification of those aeroplane. Such a more proportionate EU system is expected to also provide options for manufacturers that are currently working under national systems, to move into the EU system.
See also: https://www.easa.europa.eu/easa-and-you/general-aviation/general-aviati…

I agree with Jozef, EASA can certify light aeroplanes under 600 kg with CS-LSA, it is a matter of showing compliance with the CS-LSA paragraphs through analysis, testing .... The European manufacturers can apply to EASA for a Type Certificate, applications are welcomed!

Thanks for your feedback all! So as I understand, 600kg MTOW can be achieved with new aircraft receiving CS-LSA status, but aircraft in the air already cannot have these increased privileges awarded post-initial approval? And is my understanding correct that the same recognition across EASA countries will exist for CS-LSA aircraft? E.g., if one country opts in and an aircraft is registered there, it has the same rights in all EU countries?

The crux of the question here is, do regulations exist which would allow me to avoid flying an older, more expensive aircraft to run / maintain but would still allow me to fly around EASA countries almost as freely as traditional GA aircraft without the bureaucracy and barriers traditional ULMs have?

Well, it looks easy to have common rules across EU like EASA but price of operations is more much different then.
The main reason for national rules for aircraft less then 600kg MTOM is very simple - price.
There are still big differences regarding salary across EU so in some regions there will be impossible to maintain aircraft operations under EASA rules - at least for big part of aviators. There are national rules out of EASA supporting mainly recreational and sport aviation - that is the main reason for special category aircraft "flying sport device" up to MTOM 600 kg. It is sort of European version of Experimental aircraft in US from my point of view.
And, of course, there is still category of real Microlight/ULM up to MTOM 450/472,5 kg - very popular in some countries due to less complicated medical requirements for pilots- for example France.

The answer to your question is in our Basic Regulations where it is is defined (in the Annex 1) that some categories of aircraft will not fall into the scope of the EASA regulations. Ultralights are falling into this category but their MTOM should be below 450 kg. Some countries took the opportunity offered by the new Basic Regulation to opt-out from the EASA system for ultralights weighing up to 600 kg.
In the EASA system, we have a certification specification (CS-LSA) for aircraft weighing up to 600 kg. An aircraft certified by EASA against CS-LSA will be automatically recognized as certified by 28 countries in Europe.
An ultralight of 600 kg approved in one country in Europe is not automatically approved by the other 27 countries.
Dominique

Thank you for the clarification!