I'm pretty sure that the reason it's not updating the current version of the game instead of considering it a different game is because of the name. If "CbDE Episode 1" was just labeled as "Captured by Dark Elves", then it might update it, but then again I could be completely wrong because I don't know a lot about technological problems like these.
I'm pretty sure the reason it's not updating is android itself. I don't believe you can "update" the android version. At least I have not found a solution on how to transfer the savegames to the new version on android!
The reason I said it was the name was because it's also happened with other apk games that I've played. It can be label as an acronym of the name, and then when it comes to updating to a newer version, the all to be installed is of a different name. I guess I'll just have to save up for a pc so I can update lol