How do I move the Cannon unit to gunpowder?

I changed the prerequisite tech and cleared the cache but it didn't work

I moved Catapult to Mathematics by doing exactly what you said, but it was in a mod. Not sure if anything changes by altering main files (which I wouldn't suggest.)

I assume you were in the UnitInfos file and simply typed in Gunpowder instead of whatever the original tech was. Should work.

Changing the required tech in the unitinfos file should work. Did you do it as a mod or in your original file?

BTW, I tired cannons at gunpowder myself. To me, that seemed to unbalance them. The AI had armies of them running around. I now have them available at chemistry and it seems to work well.

I modded cannons to be available with gunpowder and engineering, and it seems to be working very well.

It works now
I changed the xml in the my docs/my games/civ4
I'm changing the cannon to chemistry too - more time to use knights

also I have castle with construction and have pikemen come earlier