https://docs.zen-cart.com/user/upgra...-zen-cart-158a

Personnaly, I use the instructions at https://docs.zen-cart.com/user/upgra..._only_upgrade/, install clone a template, install bootstrap, clone responsive_classic, clone bootstrap, add mod list, and proceed from there to use mod list to identify/rectify those things mentioned in the warning.

Admittedly, I'm probably more familiar with "where the dead bodies are buried" than most but, it's not rocket surgery.

Once all is working, I start adding mods knowing the missing or hanging on configurations are fixed.