App optimization for WP8.1

Developers will need to do quite a lot of work to optimize their apps for WP8.1, making use of all the new features and changes in the OS. I'm not a developer, but I guess that the only proper way of making a WP8 app fully WP8.1 optimized is to rewrite the whole thing.

So my concern is, how long will it take until there will be a good amount of WP8.1 optimized apps? Right now we're all pretty excited about the promising new feature of the OS, but will we won't be able to enjoy a lot of those features as long as the apps don't make use of them. I guess it all depends on how much work goes into developing a WP8.1 optimized version of existing WP8 apps. What do you guys think?