Hi and thank you for the suggestion, John.
What I did and solved my issue was using the following command:
UPDATE wpy4_postmeta SET meta_key= '_venue_custom_slider' WHERE (post_id = '29057' AND meta_value = 'field_582efaa778bfc' );
How did I do it in bulk? Good question. To get the ‘post_id’ (e.g. 29057) what I did was to:
1. Display all posts in the WP Admin > Posts area (Number of items per page: 999)
2. Filter by category (e.g. Venues)
2. Show Page Source
3. Copy/Paste the source code in the text editor (e.g. Atom)
4. Use the bulk search function (Find All) to search all my post IDs (e.g. ‘https://www.example.com/wp-admin/post.php?post=’ and selected only the string after that, next 5 digits in my case)
5. Paste that into an empty file, one on each row
6. Copy/Paste the mentioned MySQL command before and after the Post ID (using multi editing)
And the ‘meta_value’ I got it from ACF > Custom Fields > Field Groups > Field Keys (this column is hidden by default but can be shown easily from the Screen Options panel).
I hope that I remember the correct SQL command as I did finish the job the same day I posted here.
Cheers