I noticed a crack in this corner of my shower bench and afraid that water might start seeping through this part so I’m thinking of removing the sanded caulk and replacing it with silicone caulk. Is it OK to only peel and clean the part that’s cracked or do I have to remove the entire caulk from end to end?
The pictures below in the link has a red mark where I intend to stop.
That does not look like the smooth surface you get from caulk. The top section looks like it was originally grouted, then developed a crack and someone laid a bead of caulk in the crack to "fix" it. The bottom section appears to be grouted too. Changes in planes should never be grouted, always caulked. Remove all that stuff, Tap the grout gently with a chisel to break it away if necessary. Use a tile prep, cleaner and then do your caulking. I've always used a 1/4" to 3/8" bead, depending on the gap.
If that change of plane was grouted instead of caulked, then you need to remove the entire line of grout and clean the entire area before recaulking—go all the way to the next corner in each direction. Heck, even if it was actually caulked, you should remove the old caulk, clean the area, then re-caulk.
The main reason why you need to remove all caulk or grout is because of the questionable adherence of the new material you want to apply on your current dried grout.
For instance, if there's grease in those cracks or seams you want to fill up, the new material will probably not adhere. This is probably more prominent in the caulk, but still.
After you remove the grout or caulk, use a degreaser before reapplying the material.
