The MSH lock is a pretty clever way of locking a gun. If you want to lock a gun.
Swapping the MSH is simple. First you need a new MSH, mainspring, and a 1911 pin set. The factory MSH has a few non-standard parts, so you shouldn't bother trying to reuse any part of it.
Your new MSH will need to be assembled using the new spring and pins. It's pretty straightforward, but compressing the spring can be a pain.
Brownell's has a decent little instructional video:
[ame]https://www.youtube.com/watch?v=0yL3-sb9_Jc[/ame]
And the 1911 forum has a dandy howto:
http://forums.1911forum.com/showthread.php?t=334332
The Springfield factory spring is 24lbs. You can go as low as 18lbs without any risk of misfires, and usually get a better feeling trigger in the bargain. Sometimes going to a lower power mainspring produces creep in the trigger, so you might want to get an 18 and a 20, just in case the feel of the 18 doesn't appeal to you. The mainspring is the first thing that the slide reacts against in recoil, so the recoil impulse will change. Most people find it smoother after going to a lighter mainspring.