Migrating app to ES6

I currently have a fairly complex mobile/webapp that does not make use of imports or ES6 at all.

I would likely to slow start migrating my app over to ES6 so I can eventually retire Blaze and increase performance.

Is this worth doing at all?
Can I gradually do this? or will it require a complete rebuild?
Any advice or design pattern I should adhere to so lower the impact of this change?

Thanks in advanced