I used to play right handed basses upside down...for about 8 years. That came to an abrupt end during my "jazz fusion" era in the late 80s when I started learning slap/pop style and found my fingers totally out of position. I went out and bought a proper left handed bass and found the transition to be much easier than anticipated, but only because I had already become a fairly good player--tried to make the change once in my first year of playing and couldn't wrap my brain or fingers around it.