This question might be redundant, but I am trying to understand the following. I am migrating VF pages to LEX using SLDS in the phase one of migration. In phase 2 we will be building the components and I need to know the effort that is put in phase 1 is of any use in phase 2 or not at all? So basically for the restyled VF page, can it be used in any scenario in the Lightning component? My guess is it's not possible, but I may be wrong and still learning the do's and don'ts of migration.
On the other hand if I create fresh Lightning pages for existing VF pages using Lightning APP builder, is it possible to launch the lightning page created in Lightning app builder to be launched from a custom button on Opportunity object? I am confused. How difficult it is for a application written and grown in last 10 years to migrate to Lightning? Any blogs or articles about the best practices, approach etc.