Migration to the new style should be straightforward, and this is one of the problems GNUN set out to solve. If you have to migrate old-style translations, see Migrating in The GNUnited Nations Manual. If the old translation is HTML 2.0 (or 3.2), you still have to take care about the inner markup. Overall, it is substantially easier than doing all of it manually.
Subsequent migrations to newer HTML standards and newer look and feel of the website are supposed to happen semi-automatically, although this manual will be updated as needed.