Unfortunatly the Pro HD .38 super has a ramped barrel and the frame is cut for the ramp. assuming you can find a ramped barrel it will be just a barrel change. I initially thought about doing this conversion and I was quoted about $300 for the work. A recent post in the ammo forum here about .38 super pointed me to www.kyimports.com , they have armscor .38 super for $6.00/50 rds. it seems to be good ammo and it works great for me, although I have only shot 50 rds of it. I have 800 rds total through mine without a single hickup. I'd say go for it, you'll enjoy it.
Go for the .38 Super if you still can. Drop in 9mm Luger barrels may be available depending on the ramp size (width) for a reasonable price, from several sources Bar-Sto, Ed Brown, Wilson, and others, plus Kimber if there is enough interest. Even if drop in barrels are not available, .38 Super is a great caliber and well worth having. .38 Super was the first semi-auto caliber to approximate .357 magnum performance, so it will be a lot of fun.
If the barrels are available then you will be in the situation of being able to switch calibers easily like revolver shooters can do from 357 magnum to 38 Special, with just a barrel change.