To be blunt, the conversion from pre-1.7.9 to post-1.7.9 deletes all player data files; there is no way around this, as converting up to the UUID system is currently impossible.
Personally, I'm sorry for these issues; I've been working with Sam non-stop to try and resolve as much as we can, and have yet to sleep from about 18 hours of traveling and 7 hours of frantic server maintenance. We're doing as much as we can, and have reached a mostly playable state. Sadly, two things have been lost:
- All Player Inventories
- All Enderchests
This will probably not be the last time these things are wiped either; if we know damage will occur, then you will be notified. Sadly, with how 1.7.9 is being rolled out; that may not be possible.
Again. Please store all valuables in lockette-locked chests! They will be safest there!
All other development projects have been put on hold for the time being; I'll be working with Sam to see if there are ways to get around the incompatibility Mojang is insisting on, rather than working on any of the planned projects. #BlameDinnerbone
Sorry for the abruptness of it all; I truly wish we are able to recover the player data files, but things simply how they are. If nothing else, let's just keep on trudging forward; soon, you won't have to constantly regret having the username: "xXxY0l0sW4gg1nzxXx#420b14ze1t"