How can I successfully upgrade to Dawn 9.0.0 with custom code?

Hi there,

I see there is a new Dawn theme out and I am trying to upgrade my store to it.

I have done a lot of custom code to the original store - I understand that this custom code will lost in the new update however I cannot even get to updating the store and then fixing it as I get these error messages.

I have then duplicated my theme gone into my code for it and reverted the theme.liquid, base.css and multiple sections and assets to the original dawn theme by manually c+p the code from a new dawn store over my custom code.

Despite doing this to every code that I can remember editing I still can’t seem to get it to be able to update. Is there a particular code I should be looking at? Any help appreciated please.
Ideally I would like to update my store and then manually do the custom code again.

Thanks a lot.