Sorry to answer this way, but...
It depends.
If your gun has magazine tube dimples, they will have to be removed or ironed out before a magazine extension will work. If the retainer for the magazine spring is plastic- well, you have some surgery to do before the mag extension will work.
If your gun doesn't have dimples in the mag tube (look at the mag tube with the barrel removed in the area the barrel ring covers), then you are indeed good to go pretty much as you described.
There should be a spring steel mag spring retainer in the mag tube that will have to come out (and not be reinstalled) as well as the original mag spring and follower. Follow the field stripping directions in your owners manual on this one, be careful as it is under some pressure even with an empty magazine and can take off when pried out- point it away from yourself and restrain it as you remove it.
hth,
lpl/nc
