This is true, but more to the point, just KEEP THE ORIGINAL PIECES.

Anything removed with a dremel tool or a sandblaster, or put on with a torch or a spray gun, cannot be undone, but ANYTHING bolted on, whether it's a seatpost, or a bottom bracket, or a crank or a wheelset, or WHATEVER, can always be removed by a future owner, and restored with the original pieces you've kept on the shelf.

Putting on new pedals, a decent seatpost, and a working seatpost clamp will have basically no effect on the value of your bike, since anybody with a wrench and 20 minutes can remove them and put the old stuff back on.

Working pedals and a nice looking seatpost will make your bike a lot more fun for you, so put them on and go ride the heck out of that bike.