The one weak link in the 450i is the notchy shifting. Have a friend try pushing you back and forth, just a good rocking motion is all it takes, then use your shifter. If that does not work then remove the seat and look just above the dip stick where the transmission linkage connects. There is a C clip holding the linkage to the shift arm coming out of the transmission, remove the C clip but be carefull they like to fly!! Pull the linkage from the shift arm then try shifting the transmission by moving the arm itself. This will also allow you to see if the shifter on the dash moves freely. Again a little rocking back and forth of the machine will help disengage the spring loaded shift fork. If you cant shift the arm at all then you might have internal transmission problems.
I had to disengage the linkage from the shift arm one day after some really hard riding at a stone quarry, had it stick in gear while shifting on a steep hill. Disconnecting the arm did allow me to unstick the shift mechanism inside by using the arm on the transmission, but still had to rock it back and forth to get it to release. If you get it unstuck, you may want to check your adjustment of the linkage while you have it disconnected. Let us know how things work out.