It seems like running this SQL on the files tables does what I need
NB I am using postgresql
update files set url=REPLACE(url,
'oldbucketname.nyc3.digitaloceanspaces.com/mysite/prod',
'mysite-stage.nyc3.digitaloceanspaces.com'
);
update files set formats=REPLACE(formats::text,
'oldbucketname.nyc3.digitaloceanspaces.com/mysite/prod',
'mysite-stage.nyc3.digitaloceanspaces.com'
)::jsonb;