To get new strings you just need to synchronize translation files after theme update.
Button Sync (Loco plugin) allows you to synchronize your translation file with updated pot file. It means if we made any changes in template files that may affect transaltion or we added new string for the translation and you updated theme you need to synchronize your translation with updated files. Only after synchronization you’ll be able to translate new strings or strings that were changed. You may lose only strings that were changed.
Also, don’t forget to backup all the files before any update.