This helped me a lot: http://www.logicspot.com/developer-blog/advanced-custom-fields-quick-hack-to-move-a-field-to-another-group/
There described just 2 actions to achieve desired result:
Using phpMyAdmin
1. Find IDs of groups you want to move fields between (From Group -> To Group)
SELECT * FROM wp_posts WHERE post_type="acf"
you’ll see all the field groups, so just remember 2 FromID and ToID
2. Find all fields of From Group
SELECT * FROM wp_postmeta WHERE meta_key LIKE "field_%" AND post_id=FromID
then just change post_id of appropriate fields to ToID and all these fields will be moved to To Group
