Zoanthids are some of the least sensitive to water parameters, in my opinion.
I have been doing no more than 10% water changes at a time. That's not extensive. I think that anything 40% or more at one time could be considered extensive.
If you ensure that the temperature, salinity and pH of new water is spot on, then it's not going to make anything in your tank worse. The reason people advise against large water changes is because some of the beneficial bacteria in our tanks is found in the water column, and changing too much water at once runs the risk of removing too much bacteria. Also, drastic changes in anything are not advised, even if you are improving the situation.
But 10% at a time is not a lot. I think that maybe zoanthids are more sensitive to stray electricity than other animals, because by the time I discovered the stray voltage in my tank, almost all of my zoa colonies had recently started declining.
They are actually starting to open up and look better, I think, or maybe that's just me being hopeful...