Hi,
klick on: Radio0-802.11G (or maybe Radio0-802.11B) in the webgui on the main site.
On the new site you click on Settings on the top.
On this website should you now have the possibilities to select the 802.11G speeds (6, 9, 12, 18, 24, 36, 48 and 54Mbit/s).
If you don't have this choice, then something went wrong with your upgrade.
I think it should run with your IOS version, but I'm not 100% shure.
Did you exchange the radio card? If so, then it should support 802.11G (if you used a new AIR-MP21G-... module) and also should work.
Patrick