You'll be surprised by how much you can learn in two days .. Knowing standard transmission (clutch, friction point) really helps but the majority of the group in my course didn't know. Only 2 out of 30 didn't pass.
They teach you one basic after another and before you know it.. You're already riding.
I did the 250-650 (650 is not SS) upgrade last year. 9000+km on the 250 gave me the confidence to ride.. You just gotta be more gentle with the power.. Depending on what bike you upgrade to.. It's usually just getting used to the power and the gearing/shifting is obviously different... I found I had to shift alot on the 250..
Every bike is different in some way.. But knowing how to ride is ultimately the same. Definitely glad I started with the 250 tho!! Good luck