[stcredzero]

You can have the best test coverage in the world backed by a great CI culture but if porting something is a huge time investment, it may still be a huge time investment.

The right way to port is this: You make a rewriting engine that does the porting for you, and you develop on that, not on the port directly. The maintenance team keeps adding features to the legacy engine, but you can absorb those changes by running your rewriting engine on the whole code base.

Then, when the ported version is passing tests and looks good enough, you switch people over to the port. I've done this. It works. In fact, I know of at least one company that has based a consulting practice around this technique.