Is there a way to switch characters in offline practice?

How can I switch characters in offline practice in Alien Swarm? That is I want to practice as the special weapons or tech character.

Other than choosing which character you want to play in the lobby screen, you can also change which character you control during the mission itself, by using the F keys; F1 will let you control the first character (the one you started the mission with), F2 will let you control the 2nd, etc.

You can use F1, F2, F3 or F4 to take over bots at any time. This is not limited to offline practice; although I don't know how you can get drones in your team, I know it is possible.