I see it working for a backend code, legacy UI systems has way more coupling so it would be better to do a complete rewrite. If you have a legacy framework A and you start replacing it with framework B, component by component, it will have to follow the practices of framework A and basically you are going to be writing legacy style code in the new framework B which is much worse than having legacy framework A. Because framework B is now written in a completely alien way and not how it was intended to be used.