Been doing that quite a few times back in PHP-land. Partial rewrites based on Domain/Model/REST endpoint and just proxying to both webapps based on which was n

Been doing that quite a few times back in PHP-land. Partial rewrites based on Domain/Model/REST endpoint and just proxying to both webapps based on which was new already. No breaking changes from the outside, either share the parts of the code that are still good (most of the business logic might be) or fork them (and then refactor and maybe you need to fix bugs twice) and after a while you can switch off the old part. Works like a charm with added benefits of being in the same language to avoid wasting time. But the base idea works even if you use different languages. It's also not for web apps, for example Apache Storm can do Bolts? (been a while) in several languages so you can also easily rewrite parts, if you can serialize your data in and out of it.