With Microsoft pushing Universal Apps for everything Windows 10 (PCs, tablets, phones, Xbox) I wonder if non-universal apps currently available in the store will continue to work on Windows 10 (at least on phones). I assume this will work, but I haven't read any official statement yet.
After installing Windows 10 Technical Preview I can confirm that WP7.x and WP8.x app work fine on Windows 10 for phones.
There was already a statement that Windows Phone 8 devices will be upgradable to Windows 10.
I'd be surprised if they were to release an upgrade that would stop previous purchases from working following upgrade of the operating system. I'd also be surprised if there weren't still phone specific apps (like Here Drive+) whilst others may migrate to being universal apps (Like Twitter already has).
From a development point of view, universal app developers are already encouraged to target the different screen sizes/formats, and the development templates default to having different layouts for each platform (although it is possible to share them if you really want)