I personally went from a Yamaha Aerox 50cc to a Honda Varadero 125 (Similar to Yamaha R125), and it really wasn't that bad at all.
Spend a few hours watching videos on youtube on how to change gear etc (link below).
Only problems I had was the quick acceleration (cause obviously you're used to using all the throttle on the 50cc), and a couple times of stalling. But after the first week I was fine.
Do you mind me asking your age, and if you're planning on getting full bike license too btw?
A lot of guys here will recommend a YBR125 as would I, but if you're like myself and prefer "sportier"/quicker bikes, then a R125 would suit you perfectly.
A YBR125 is good for about 60-65mph and after that it just doesn't feel "safe" riding it (vibration), not to mention wind..
http://www.youtube.com/watch?v=hMxKMa-WPvo
Thanks for the advice. I'm 17 btw.