Rebase an existing theme onto a fresh / updated copy of the theme framework

migrate-theme.md

git clone <a copy of sage theme>
git add remote temp location@of.old.theme.com:/var/www/site/wordpress/wp-content/themes/build #original / old theme
git fetch temp
git checkout temp/master -b wip #checkout a copy of the old theme
git remote add https://github.com/roots/sage.git sage #fetch fresh theme
git fetch sage
git checkout sage/master -b sage #checkout a new branch
git rebase wip #rebase wip and sage together to create a new updated theme

#If conflicts arise:
git status #see which files are conflicted and edit the conflicts as necessary
git add -A #add all changes for rebase
git rebase --continue #continue with rebase
git rebase --skip #in rare cases where git add doesn't do anything / no files are required to be added

#Recompile your theme - run bower install, npm install, gulp etc