While you can "install local apps" that is only for where you put the original package on the phone. Apps that are installed can
only be used in that install of Windows, or one that it is upgraded to. You can not "re-install" already installed apps from your SD card. So once you downgraded back to WP8.1 you have two choices:
- Wipe the apps on the SD and download them from the store again.
- Wipe the SD and put the original app packages on the SD card to re-install. (This is called side-loading, and requires turning on the developer features. And on top of that I think is limited to ~5 apps.)
In other words, you have to wipe the apps and re-download them. You can't move an SD card from one install of Windows to another and use the installed apps that it contains. (That is true of WP8.1 as well.)