Change a custom field on all posts Posts not showing up!

Hi all.

I’ve just switched theme and am having some major problems!
My old theme needed a custom value of ‘image’ to show images up on the site.
This new theme needs custom values of ‘lead_image’

Is there a simple way of changing them all from image to lead_image rather than editing each individual post!? (which i’ve done for the first few)

also, since changing theme, when clicking on a post nothing comes up any more – the url doesn’t seem to change?

Any help really appreciated!
ref: https://www.chrissayburn.com