Since it worked on Ground Zero and I'll just tweet a little bit with the switching weapon with first expansion as well. Here it is:

// Alias scripts

alias blas "bind 1 trap; use Blaster"
alias trap "bind 1 blas; use Trap"

alias rock "bind 7 phala; use Rocket Launcher"
alias phala "bind 7 rock; use Phalanx"

alias hyper "bind 8 ion; use hyperblaster"
alias ion "bind 8 hyper; use Ion Ripper"
// weapon binds

bind 0 "use BFG10K"
bind 1 "blas"
bind 2 "use Shotgun"
bind 3 "use Super Shotgun"
bind 4 "use Machinegun"
bind 5 "use Chaingun"
bind 6 "use Grenade Launcher"
bind 7 "rock"
bind 8 "Hyper"
bind 9 "use Railgun"

// EOF

Create an Autoexec.Cfg and enjoy it