WP7 is not an upgrade path for Symbian users. They can customize almost everything on their phones while in WP7 you can customize almost nothing. Symbian users who are used to widgets, themes, multiple homescreens and Java apps will feel constricted with WP7. The natural upgrade path for Symbian users is Android.
Then why does Elop still fear to sell N9 in major markets alongside Lumia?