Thanks @pboislard33288 and @sacwebdeveloper! This was seriously annoying... I can't believe how badly one can fail on this "simple" magic. :)
THE SIMPLEST WAY to resolve when "october Builder" doesn't recompile DB Migrations is to go into the "system_plugin_history" table and manually remove your entries for (that build of that plugin) then run CLI command: "php artisan plugin:refresh author.plugin".
or more simply, change the sequence-numbering of the entries in the "plugin/updates/version.yaml" file from 1.0.1 to 2.0.1 and then run CLI command: "php artisan plugin:refresh author.plugin".