Do you mean they don’t appear automatically in the old posts or you can’t see them if you go to add images to the gallery field?
I don’t think there is any easy way for you to accomplish what you want other than editing each old post and adding the correct images to the gallery field so that the data gets saved into the database and you can display it in your theme. This can possibly made easier if you set the gallery field to show only images uploaded to the post.
Of course there could be a way to write a php script that would fetch the old images from old posts and alter the post custom metadata in the database to contain the gallery field code…
I don’t know exactly how you have your previous content set up but one another option could be to code your theme files in such way that if there is a new gallery field with content, display it but if not then use the old way and use php to alter how that data is handled…
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