Current apps will be working. Windows 10 is backwards compatible as most of the apps/games in the windows 10 store aren't true windows 10 apps. I'm just not sure if windows 8.1 devices can run true windows 10 apps. Not all devices will get Windows 10 so not sure if they will still maintain the windows 8.1 store. Or if windows 8.1 devices can still run apps specifically built for Windows 10. It's really a mystery how they will do it. Maybe the developer tools let you also publish your app as 8.1 for windows phone. And it just does all the work. We will see when it comes out.
I know I would not publish my apps/games to a build version of windows 10 mobile and risk the low ratings from all the people having issues that may be related to the build. Plus API calls may change among other things. We don't even have all the features in our build so how are developers supposed to take advantage of missing features? Just give it time. Oct 6 you will have more info.