showerhead

Help please. Wife heard a clunk in the shower and then the pressure in the shower wand was lower (but still functional). We have calcium rich water (as opposed to hard water, according to the water source it's not hard...probably due to lower levels of magnesium) but I'm not sure it would be...

This dilapidated 60s house in still inhabited, and there's an effort underway to switch out the shower head for a handheld to accommodate an invalid. I don't have much experience here, and I'm afraid this ancient plumbing might just break if I try to unscrew this piece on the end of the pipe...