Did you do a gravel vac BEFORE you started the treatment?
If so, I'd suggest you hold off on the water change while the meds are doing their thing, and complete a LARGER water change at the end of the course of meds. - If you normally would be doing a 25% water change, up the change to a 50% change on Sunday, or whenever your treatment is finished. I'd mention that using a smaller dose sometimes means a longer treatment, so beware of that.
Doing the water change isn't a bad thing, in fact it can be quite helpful, as during the water change you will want to gravel (sand) vac, and try to pick up any trophants that are sitting at the bottom. These will eventually burst and release the free swimming form of the parasite (which is the only one that can be killed by the meds). Less of them, means less chance of the meds not working.
Did you increase the temp of the tank? The warmer the temp the faster the lifecycle of the parasite and the quicker it reaches the free swimming (killable!) phase of its life cycle.
BTW, since you are having the dose, I'd double the length of treatment, just to be on the safe side. Ich is one of those things that you just don't mess around with. It is dealt with fairly easily, compared with some diseases, but if you don't fully irradicate it from your tank, you will be back to square one in no time. Might as well medicate a bit longer than necessary than not long enough. Just because you may not see any more sores on the fish, doesn't mean the other forms aren't present.
