I have spoken about this with my league operator many times. If you have an established 8 ball S/L, you will start at that level in 9 ball. The LO always has the discretion to raise the players skill level on the spot. A blanket rule of raising it one level for all players in 9 ball indicates a league operator that knows nothing about pool. I have a girl on my team that is a relatively competitive SL 4 in 8 ball. She wins through good strategy, table management, and shot selection. These variables are significantly less important in 9 ball, where all out cueball movement is much more significant. As such, she can only play as a 2 in 9 ball, and often loses.
I was a fairly strong 7 when I joined 9 ball. I spent about 3 weeks a s a 7 and went up to an 8. I spent about 3/4 of a season there, then went up to a 9. I would have moved up faster, but I had to learn how to play "APA 9 ball", which is a very different game than regular 9 ball.
Hope this helps,
Kerry