First calibrate your test kit to make sure it's reading correctly.
Check your tap water.
If you are dosing KNO3 fertilizer, then stop.
If the high nitrate readings are correct, try eliminating/great reducing elements (feeding, etc.) that add to the nitrogen load (anything that will convert down to nitrates, making more)
Inhabitants can actually tolerate very high levels of Nitrates for quite a long time (several months, of course levels within reason and depending on species).
Since you are not seeing any signs of stress, I wouldn't rush things. Many people say large water changes can be harmful to shrimp, so the large (80%) water change will most likely be a lot more harmful than staying in high nitrates levels waiting for you to do smaller water changes.
Large water changes in itself aren't really harmful to beneficial bacteria, unless you have significant temperature, pH, etc, differences. So if all other water parameters are pretty much the same, a large water change wouldn't be a problem really. Do make sure you use enough dechlorinator/water conditioner though. Fish do appreciate smaller water changes over large water changes, pretty much for the same reason. Even if fish seem to really appreciate large water changes even if the parameters are pretty different, the adjustments their body is doing to readjust/readapt to the water parameters does put a bit more of a toll on fish's insides (shrimp and certain fish being more sensitive than others). Although not really noticeable unless a really drastic change.