Transfer stats from one player to another in minecraft
Solution 1:
First of all: Stats are now per-world.
You can find your related stat data in /[Minecraft Directory]/saves/[Save Name]/stats/[guid].json.
Just rename the file with your old account's GUID to the new account's GUID and you'll leave off as if you were the previous account.
If you did not backup your worlds... This unfortunately means you have lost your data. There is no way to get it back, like there is no way to go from a burnt piece of paper back into pristine condition.