I have upgraded to 1.8 but I dislike the new editor, mostly because it destroys half the BBCode, especially that my forum is very based on heavy custom text formatting. On the previous editor it worked fine, but 1.8 one is broken. How do I return to the old editor?
Short answer: You can't.
Long answer: You can't, unless you're willing to put in the work. It would require you to convert the old editor to jQuery from Prototype, and update various templates and files, plus maintain the the changes you made. To 'return' to the old editor would require a lot of effort which could be better spend reporting the issues and getting them fixed.
Well I guess the best solution for the new editor would be to process the MyCode as it is, rather than trying to "correct" it in any way because it most of the time breaks everything completely.
Well, we can't do much since there are some bugs in the SCEditor core, like this one:
https://github.com/samclarke/SCEditor/issues/356 and we would have to rewrite it, which is not a good idea since we don't have enough activity to fix bugs and introduce features in our core as fast as expected by some.
Also, you can put the editor in less buggy source mode by default or disable it completely in User CP -> Edit Options.