Originally Posted by
Grizl' Bjorn
First of all- cool to see another Australian!
Second of all, copyright law is a complex area and I can't answer your question. What I can say is that 'how much do I have to change' may not be looking at it the right way. You'd have to talk to a lawyer.
Thirdly- even if it is very different from 5E, one option is just to publish it under the OGL through the DM's guild and brand it as a 'reinterpretation' of the 5E rules. I doubt Wizards of the Coast would object- it's not like you're taking anything away form them.