These are the famous Remington 870 Express "dimples".
Remington introduced these on the Express line strictly to allow making the 870 Express cheaper so it could compete with the cheaper to make Mossberg and Winchester guns.
There are a number of methods of dealing with the dimples:
The easiest is to use a 3/16" or 1/4" drill to drill the dimples out.
A gunsmith with the tooling can use a shotgun dent expander to "iron" the dimples out.
One poster on another forum used a socket wrench socket of the right size to "iron" the dimples out.
Another is to use a Dremel tool to grind them off.
Whatever method is used, you have to make SURE the entire area is sanded, ground, filed, or polished TOTALLY smooth.
There can be NO traces of the dimples left, or the follower or shells may hang up in the tube.
For all intents and purposes, the conversion is permanent.
Without the dimples and the plastic magazine plug, there's nothing to keep the magazine cap from unscrewing.
You CAN convert back by buying an old style magazine spring retainer.
This is a spring steel cup-washer which retains the magazine spring, but unless you screw the cap down TIGHT with a pair of pliers, it can vibrate loose under recoil.