It depends. What parts of Bootstrap are you using for the site? If you're just using the grid system, there are plenty of drop-in replacements that work similar

It depends. What parts of Bootstrap are you using for the site? If you're just using the grid system, there are plenty of drop-in replacements that work similarly. If you're using form styling, typography styles and other Bootstrap helper styling, then it gets a little more tricky. Approach it like you would any rewrite, tackle each part of your application as a separate modular component. Do the homepage, then static pages and so on until you have moved every page over from Bootstrap.