[SwissXC90]

You may be snookered

I have converted Japanese BMWs from Japan market spec to European market spec in the past.
But in the early days (1995 to around 2007) this was easy as the platform was built to be multi-market and I could change the navigation drive and it would work. the other items were programmed by BMW to talk to both Japanese and Euro nav, which incidentally had different network addresses.
A little bit of reprogramming was required but not much
But even then, the car remained clearly labeled, both physically and electronically, as a Japanese market vehicle. With all the Japanese oddities that they have.
Re-branding the car to be a Euro market car was not easy, and a Dealer could NOT do it, as it meant changing the factory order, and that was NOT permitted or possible with their tools.

Skipping forward a decade, and the software and electronics have gotton much more complicated and much much more integrated.
Sometimes the conversion requires hardware changes, which for nav units is often the case due to the different navigation hardware suppliers required for Japan due to the different map formats and due to the different characters in the language. It gets really complex.
Some things can be as simple as a software change eg for a radio, to change from Japan to Euro mode for the radio.

But the navigation is ALWAYS much more complex that the radio, purely based on map material.

Then add into the picture the complex network architecture, and the special Japanese things like Japanese toll booth devices, radio data receivers, special phone systems and more.
A modern vehicles network often needs everything in place to work properly

SO: while I am not saying it is impossible, I am saying it is not easy at all.
The simple way is to fit new virgin hardware and have it programmed by the dealer.
This is what the dealer is offering and is the only thing they can do.
They have no other option at all.

Nobody that I know of yet offers re-virginising for Volvo control modules, which takes a 2nd hand unit, and reflashes it to appear as a new, unused, never fitted unit. A virgin.
Some people can do this to VW, Skoda and Audio systems.
But not Volvo, Not that I have ever found.

SO: you may need to say to yourself "I saved XXX GBP by importing a 2nd hand Japanese market vehicle into the UK, and now I can afford to spend YY GBP to convert it to the Euro model".
Hopefully YY is less than XXX.
If the numbers work for you, great.
If they don't, bad luck. That's the risk with foreign market vehicles.

Following my experience a decade ago, I would NEVER personally buy a foreign market vehicle. The hassles with headlamps, headlamp washers, emissions, safety systems, bumpers, number plate mounting sizes, radios, navigation, telephone, and then also spare parts all make it a right royal pain in the rear bumper to own such a vehicle in a market that it was never intended to be in.

PS: don't expect any "goodwill" from the official Volvo channels. They will tell you very simply "The vehicle functions correctly for the intended market of use. Conversion between market variants is not possible and not supported, due to the differences in technology, safety, emissions and regulatory compliance".
Just the regulatory compliance issue is a big enough card for the dealer to play for them to totally avoid the issue.