It's a matter of putting together a second top end:
Complete slide
Complete barrel
Bushing
Guide rod
Recoil spring
Plug
and possibly a second slide stop.
I have one with three tops: 45ACP, 40S&W and 9mm.
They all work fine with the same ejector on the frame.
The 9mm requires a different slide stop while the 40 and 45 use the same one. (This only matters if you care whether or not the slide locks back on an empty mag)
Speaking of mags, you'll need those too.
I have no idea what's required in the area of fitting the new top end to your frame and whether or not it would be more expensive than a whole new gun.
A couple of advantages are:
Your grip, feel and trigger will be the same.
You only have one gun to register, acquire an ATT for, lock up, etc.
If you use a full length guide rod the whole package can be kept assembled in one neat unit when the 45 top is in use.