I feel that it's not good to have the other existing core devs to not be part of this new project that will be replacing the system that they will ultimately be maintaining. Those members are eager and passionate to contribute to innovation and I feel not allowing that is harmful.
I recently read this article about how Facebook had their core team working in parallel on an innovative project. https://hbr.org/2015/06/an-inside-look-at-facebooks-approach-to-automation-and-human-work
I'm wondering how to do something like this on a tactical level, especially in an agile scrum shop?