Debian Jessie will stop receiving security updates in 2020. Cloud Services will shutdown any remaining Jessie instances come January. https://wikitech.wikimedia.org/wiki/News/Jessie_deprecation
This should be as simple as following the instructions at https://wikitech.wikimedia.org/wiki/Tool:XTools then recreating the web proxies to point to the new instances. There will be an unavoidable period of brief downtime.