majorly changed template based on open source template or purchased template, ok to upload?

Can we build our template ( with major changes ) on open source templates or purchased templates? Would that be accepted upload that we can sell on themeforest?

No! it’s never acceptable. If you want to create a family product (like: HTML to WP theme or Joomla Template or React Template) then you must have to take permission from the author of the purchased template.