I can't imagine this converter plate is going to look great, why do they want to convert it? If going from something that big to GU10, the light output in comparison may be terrible. Is it to convert to LED? If so what is the price for an LED lamp for existing fitting?
If it's to look nicer, I'm afraid that's a fail for me, it will look like a mess with a conversion. Would be better to spend a little extra and skim out the ceiling, its hardly going to be that expensive. I know in this area its around £40 for it.