Probably more stress and aggravation resetting a poorly maintained tank - since there is existing livestock it has to be done quickly and there is little margin for error.
How much work to maintain the existing tank? This depends on the condition of the rock in the first place, if it is riddled with unwanted organisms and nitrate/phosphate, then it may take some constant attention to get it to a fairly stable state. Also try and avoid the ph altering additives that go with the tank, there are better ways to stabilise the water.
At least starting from scratch you know what you are getting and don't have to work around what is already existing, and you can take your time to get all the parameters right before you even attempt to start stocking it with fish that you have researched well and chosen for yourself.