that seem more significant than extra hands, like really important things like vriska killing people or something. Are these retcon things completely changed? as in, the original panels? is it still possible to read it how it was? I'm confused.

There was a another retcon after that. Terezi had John use his retcon powers to go back in time and stop her from killing Vriska. You can still read the original timeline but once John finishes the retcon you’re switched over to the new timeline. Last page before the gigapause is here.