Is it possible to update elements of Bootstrap sass

I've been tasked to update a few web pages on my companies WordPress website. The new design they've asked for looks like standard Bootstrap Grid functionality.

The website it currently using Bootstrap v3.3.7, seen in the _bootstrap.scss file.

Although this version has a working grid system, it's missing some useful features like:

   <div class="col order-last">Last</div>
   <div class="col order-first">First</div>

I've added a CDN link to Bootstrap v4.3.1, which makes the order-last/first classes work and confirms it's an issue with the bootstrap version. But it overwrites a load of custom styling.

Is it possible to update parts of bootstrap so I can preserve all the styling alterations the previous developer made?

Or, would I have to replace all the bootstrap files for v4.3.1 and try to replicate the previous developers work?