I wouldn't define that is improv if all you are doing is tinkering with an existing line of music. I'd define that as embellishment. "Improv" in classical music would be in the form of cadenzas, where the line is not explicitly defined. Nowadays cadenzas are usually defined for us and written out, but it's well within the bounds of propriety to define and create our own, or alter the line to suit our own style. In the version of the F.J. Haydn concerto that I performed in HS, the written cadenza for the 1st movement had a line that I either didn't particularly like, or I didn't play it well because of how it layed, so I changed it. Done. Even that wasn't really improv though because I did it the same every time. True improvisation is just that - creating a melody on the fly that is built around the chord progression of the piece of music. That's my 2¢ on it anyway.