Can I select the entire second or third level in an org. chart to change the font, en masse?
Also can I cast the org. chart in stone to prevent anything changing? Poor choice of words but I hope the readers understand.

If you've used styles to distinguish the levels in the chart, then all you need to do is modify the style definition. As for locking content, I believe there are several different options … that other people on this forum know a lot more about than I do :-}