Update option on existing links

Are you comfortable enough if I give you an SQL command to run on your database to batch update all of the targets at once?

Something as simple as this should do the trick:

update wp_postmeta
set meta_value = "_top"
where meta_key = "link_target"

The meta field link_target is probably going to be unique on your site. If you want to be sure, you could use the following query to first check what are the current values in that meta field:

select distinct meta_value from wp_postmeta
where meta_key = 'link_target'