How to implement Bootstrap 4.0 into a 2003 project "gradually"?

Question

Layout

There is an old project, you need to “modularly” cut bootstrap 4.0 into it.
The option to change everything at once does not fit, the option not to tie too.
How can you inject it into certain parts of the application (in the spirit of iframes, but preferably without them) so that in the end it’s painless to switch to new and fresh styles? Maybe something in the spirit of the Jquery.noConflict approach is there?

Answer 1

Connect it with the first file of existing styles and see how much it will ruin the layout and fonts, then try with your styles as much as possible to return the site to its original state and if it works out, then you can safely typeset the new functionality on the bootstrap

Answer 2

even strange things are advised.
Just create a new header with bootstrap and hack up the old one on each page by rewriting the page.
So gradually rewrite everything.

Answer 3

1. Plug in the TB styles at the very top.
   
2. Incorporate TB styles gradually into HTML and eliminate common rules from old CSS.