I just tried an oscillating tool with a carbide blade to remove grout from between wall tiles in a less visible bathroom area, and despite starting very carefully I'm chipping tiles all up and down their length.
The grout line is between 1.9 (5/64") and 4 mm (5/32") wide, usually 3 mm (1/8"). The area I tried has around 2.9 mm wide grout at narrowest. The blade, including the carbide grit coating, is 2.6 mm (7/64") thick at maximum. I aimed for 2 mm reduction in grout depth. It went OK for the first couple of passes, then the next pass I chipped the tile all along its length, even though I didn't slip that I could notice, presumably because of the very narrow clearance between blade and tile.
Few web stores mention blade thickess and the thinnest I've found on the web or locally is only 2.3 mm (3/32"), so not much different.
I don't see this going well, does everybody who has success with these tools have wider grout, or a narrower blade?
Another problem is that some of the grout is level with or even slightly above the tile (maybe 10% of the grout line, but maybe 1/3 of tiles have some grout like this). Where it's not, it often fills the gap fairly high - it's 1 mm below the tile surface at most. It's hard to see how I can grind that out without chipping tiles.
Is it a good idea to regrout this at all, or is it a fool's errand?