Thanks Gail_Blitzy! Micromesh is a little button in the Geometry tool bar that replaces each polygon on a polygonal object with a different mesh of your choosing. It's really awesome!
I've been told someone has emailed them to figure if they mean polygons or triangles. They mean triangles. I thought it was a little steep for polygons, and that confirmed it.
Either one-by-one in pairs, or shift-drag the edges of both sides of the polygon to make nev faces on each side, delete the polygon and bridge the edges:
If there's a polygon in between the verts you're trying to weld it won't let you. If this is the case, delete the polygon/s that's in between them and then try welding the verts again.
cryrid is right. Micromesh is completely polygon-dependant. What you could do is use the ZRemesher function to generate a mesh with less polygons whilst preserving the silhouette.