First take a db backup!
Second, in your question you mention the same meta_key, so made the following assumption:
The meta_key you want to keep is “ehp_citation”
The meta_key you want to change is “ehp_citation_old” * so make the correction accordingly
Then you can try something like:
UPDATE `wp_postmeta` AS pm_to_change
LEFT JOIN `wp_postmeta` AS pm_we_want ON pm_to_change.post_id = pm_we_want.post_id
SET pm_to_change.meta_value = pm_we_want.meta_value
WHERE pm_to_change.meta_key = 'ehp_citation_old'
AND pm_we_want.meta_key = 'ehp_citation'
AND pm_we_want.meta_value != ''
also make sure you change the db prefix to mach yours.
To explain,
1) we say we want to update the wp_postmeta table and for reference we give it a name: pm_to_change
2) we make an LEFT JOIN to the same table (wp_postmeta) but this time we refer to it by: ‘pm_we_want’ and also we say that we want the data for the same post_id
3) here we say what to change. we want the meta_value of the pm_to_change to be SET to the meta_value of the pm_we_want
3) Finally we specify from which meta_key we want the above values.
So we want the meta_key of the pm_to_change table to be: ehp_citation_old and the meta_key of the pm_we_want table to be: ehp_citation.
Bonus 1: The AND pm_we_want.meta_value != '' in the end also checks if the value we want, from ehp_citation is not empty. Which means that if it is empty, the we keep the old ehp_citation_old
Bonus 2: Run this SELECT first to check that we got the correct data:
SELECT pm_to_change.meta_value AS 'Change_This', pm_we_want.meta_value AS 'To_That'
FROM `wp_postmeta` AS pm_to_change
LEFT JOIN `wp_postmeta` AS pm_we_want ON pm_to_change.post_id = pm_we_want.post_id
WHERE pm_to_change.meta_key = 'ehp_citation_old'
AND pm_we_want.meta_key = 'ehp_citation'
AND pm_we_want.meta_value != ''