The spring can be pulled/twisted off the plunger. (I find twisting clockwise works easier for me.) Yes, the open coil end will probably lightly score the plunger.
The new spring can be twisted/pushed onto the plunger until it snaps over the rebated edge near the the collar.
I use a good quality cotton swab, making sure no fibers are left inside the channel to interfere with plunger & spring movement. The long wooden stick swabs sold by Brownell's seem to resist losing fibers better than most cosmetic swabs. (And "budget" cotton swabs can fall apart faster than you might expect when used to clean guns.) You want to make sure the channel is left clean, dry AND free of cleaning materials.
If you order from S&W the spring & plunger are:
061100000 $0.51 DRAWBAR PLUNGER
105760000 $0.51 DRAWBAR PLUNGER SPRING
They might send you one of each if you call and ask. The postage will cost a lot more than the parts.
The current drawbar plunger spring is actually listed as "rev A1" on shipping lists if you order it from S&W. Unlike other small springs used in the guns, the drawbar plunger spring is an "open" end spring, meaning the coils aren't machined down and "closed".
FWIW, for the sake of trivia, at one point during ongoing 3rd gen production they made some changes to the drawbar plunger and its frame channel.
The plunger received a tapered point, instead of the flat point of the original plunger. The tapered point reduced contact between the front edge of the plunger and the inside of the coils during spring compression (during trigger pull).
The mouth of the frame's channel/hole also received a slight bevel, to reduce resistance between the edge of the channel's mouth and the outside of the spring coils as the spring was compressed. Less bumping of each spring coil against the mouth of the channel during spring compression.
The front of the channel also received a small hole machined into the top of the channel, at the front, to help relieve any pressure build up as the spring was compressed, especially if some solvent, oil or CLP had somehow found its way into the channel and covered the spring. (The channel is normally supposed to be clean & dry).