Never heard of such a thing. Usually just straight swap and be done with it has always been my case. How many miles on that engine? If it's not a very fresh engine, you may be setting yourself up for leaks.
In my case I knew the engine had over 200,000 km on it when I got it and I was ready for the worst anyway.
I went through 4 oil changes every 1,500 km when I first got the car and the oil came out pitch black in that time. After the 4th the oil stayed clean for 3,000 km.
Then I decided to switch to a full synthetic.
That is how I did it, 1 litre more each oil and filter change after 3,000 km. The old oil went black around 2,000 km the first time and the oil pan leak started to stop. By the time I got to complet synthetic the leaking had all stopped and the oil stayed clean for over 4,000 km.
On a recent trip of 4,200 km I used 500 ml of oil.
Your engine has been treated like a baby so with only 48,000 km I would just switch. If you are planning to continue to treat it like a baby and not drive it then just use Dino oil.
I don't know if there's any hard data on it, but every experienced mechanic I've ever asked has stressed that you should never mix synthetic and dino oils. (Yes, I know that there are some half-and-half blends that come like this.) If I were you, I'd just switch cold turkey.
BTW, if you change your oil & filter religiously every 2,000 or 3,000 miles, you might as well stick with regular oil. On a daily driver, the extra cost of the synthetic will outweigh the benefit, IMHO.
If you do a full switch over change your oil after about 1000 miles, or run seafoam with dino oil for the same ammount before. The synthetic is a much better cleaner and will loosen any of the gunk in your engine and could clog the filter or passages some. Clear it out so you are not driving 3000+ miles on it.
I switched my 1990 560 SEL with 90,000 miles that had been using 10 40 petrol based oil each and every 3000 miles since new to a full synthetic and within the next 2000 miles had to replace both valve cover gaskets and pull the transmission to replace the real main seal. I would continue to use the petrol based oil in any engine that had been using it unless the engine had be apart and had all new gaskets/seale replaced. The synthetic product my be better but IMHO it will cause oil leaks......mayolives