Page Builders that leave code behind are a real pain. I am assuming it left shortcodes behind, which is typical. I don’t have a fast and easy solution for you, but I do have one.
You can do this before or after the import. I usually do it after so I’m not wasting time fixing things that weren’t moved.
Install the Search Regex plugin. In the plugin itself, you can set up regex commands to do search and replaces on the legacy builder code. This is a long process, since those items can include a lot of different shortcodes behind.
First you want to find all the short codes, but that is fairly easy with this plugin since you can search for the prefix being used and get all results.
Besides identifying the shortcodes you want to replace, you need to create regex replacements with wildcards for maximum impact.
For example, here’s a Visual Composer <hr> that I removed using this tool.
Original
[vc_separator color="custom" accent_color="#4899ac"]
My Search
\[vc_separator(.*?)\]
My Replacement
<hr />
You also get to preview the changes before you run the script, so you don’t need to be anxious about unexpected results. Also – keep track of all of the replacements you’re doing so you can repeat the process if something does go wrong.
I’ve done this same method to remove Divi, WPBakery, and Visual Composer. Hope this helps, sorry I don’t have a faster or simpler solution for you.
Here are a few examples for Visual Composer just to help out:
My Search
\[vc_single_image image="(.*?)"\]
My Replacement
[image id="$2"]
My Search
\[vc_custom_heading text="(.*?)" font_container="tag:(.*?)\|.+?"(.*?)\]
My Replacement
<$2>$1</$2>
And then I did these two absolutely dead last with no replacements:
My Search
\[vc(.*?)]
My Search
\[/vc(.*?)\]