Translations:Help:Extension:Translate/Page translation administration/89/en

This is manual work, where you have to open the old translation page and copy and paste translations from there to correct translation units in the new system using the translation interface. For this you need to roughly know which part of the translation matches which part of the old text (and hope they match). You might want to consider marking all the migrated translations as needing update by prepending the string   to the translations and have a translator look at them. Once migrated, you can delete the old translation pages if they are not using the same naming convention (or you could have switched them to it before migration). Once all pages are migrated you can also remove old language navigation templates.