As long as you add your water conditioner and temp match the fresh water before pouring it in your tank water changes are not going to affect your cycle. The bacteria doesn't live in the water. It is growing on your filter media and on all the surfaces in your tank.
The beauty of a sand substrate is you don't have to deep clean it like you would gravel. All the waste sits on top. In my case it doesn't accumulate on top at all. My filters pull all of it out of there. But if in your case you find it is accumulating just swirl the vac tube above the sand and pull the waste out. Hold the tube at least 1/2 an inch above the sand.
I both do and recommend changing out half the water each week and then once a month change out even more. Fresh clean water is the very best thing you can give your fish to keep them healthy. I have to think big weekly water changes are the main reason I've never had to deal with sick fish. Most of the ones I lose, I lose from old age or an occasional jumper.
First, squeeze out and rinse the filter pad or sponge in a container filled with tank water. (You can use the water you just removed from your tank.) Using tank water helps retain the beneficial bacteria that keeps your water clean. Washing it in the sink with regular tap water will kill, or at the very least, disturb, that good bacteria.
If it’s time to change your filter media, aka the carbon, ceramic or other products that go inside your filter and do the actual filtration, this is a good opportunity to do so. Check the instructions on your filter media packaging to determine how and how often you should change it.
Make sure you also rinse the filter tubing and any other parts of the filter that come into contact with water.
Important note: If you’re doing a heavy cleaning (for example, if you had to scrape persistent algae off of your tank’s walls or needed to use bleach to disinfect your decor), skip this step for now. A heavy cleaning means you’ve already introduced a lot of change in the tank, which can be stressful to your fish. Doing a filter cleaning on top of that adds even more change to the process, which can be detrimental to your tank inhabitants. Move on to the next step, and plan to come back and clean your filter in a week or so. Remember: Regular light cleanings can make this process easier both for you and for your fish.