There is currently no direct method to upgrade zone density in popular city-building game, Cities Skylines.
It does not mean it cannot be done. In the following workaround, I have discussed how to upgrade zone density, but do note that you will lose some population during the whole process.
First things first, did you ever try and use a high density residential or commercial zone on a lower density one? In case you did, you must have noticed that by doing so, nothing happens as Paradox Interactive has not made it possible.
Coming back to the said workaround, in order to do so; first you need to de-zone the residential or commercial area you need to upgrade or downgrade. Do note that doing this will cause all un-zoned buildings to disappear.
Once you have de-zoned your desired choice of area, you can start re-zoning it with the Higher Density version. This is pretty much everything you need to do in order to upgrade/downgrade a zone.
Like I have mentioned earlier, you will certainly lose a chunk of your population but it is not much of an issue.
Did this method work for you or are you in need of further guidance? Let us know in the Comments Section below.