HI I have not done it but I have a least 5 etzs125 and as many etzs 250/251. The 250 is a larger moteur and the rear wheel and swing arms are different. The etz 250 wheel is also wider. The difference is about 2cm.It is a lot of work for nothing. Both the 125 and 250 are nice bikes.Sell ,the 250 moteur and buy a 125/150 moteur. Many of the other etz parts are common but not the moteur and the rear end in general. Joseph Schock in Paris