Yea, for what you're wanting to do DF's method would be your best bet. It's join_class though. Unless joinclass works too.

hrug:
scout.cfg:
bind f "join_class spy; join_class scout"
soldier.cfg:
bind f "join_class spy; join_class soldier"
pyro.cfg:
bind f "join_class spy; join_class pyro"
demoman.cfg:
bind f "join_class spy; join_class demoman"
heavyweapons.cfg:
bind f "join_class spy; join_class heavyweapons"
engineer.cfg:
bind f "join_class spy; join_class engineer"
medic.cfg:
bind f "join_class spy; join_class medic"
sniper.cfg:
bind f "join_class spy; join_class sniper"
spy.cfg:
bind f "join_class pyro; join_class spy"
I use my numpad to switch classes. 1 is scout and 9 is spy, just like on the menu.
bind kp_end "join_class scout"
bind kp_downarrow "join_class soldier"
bind kp_pgdn "join_class pyro"
bind kp_leftarrow "join_class demoman"
bind kp_5 "join_class heavyweapons"
bind kp_rightarrow "join_class engineer"
bind kp_home "join_class medic"
bind kp_uparrow "join_class sniper"
bind kp_pgup "join_class spy"
bind kp_ins "changeteam"
bind kp_del "changeclass"