The bug

If any blocks are placed by the player in the chunk directly preceding the 30 million block boundary in a version before 1.8, and the world is loading in a version after 1.8 (e.g. the latest version), those blocks will become permanently inaccessible.

How to reproduce

A world from 1.7.9 is attached to this ticket.

1. Load into the world in 1.7.9
2. Confirm the diamond blocks are accessible
3. Load the world in the latest supported game version

Expected results

You would be able to access and retrieve your precious diamond blocks.

Actual results

Locked away forever because of the world border.

How to fix

Move the world border out to exactly 30 million blocks, where the proper world boundary is.

While Dinnerbone has stated in a comment in MC-53390 that the world border is intentionally placed a chunk before the 30m limit due to "a sudden "drop" off the edge of the world", it is unclear what this actually means;

• if it refers to terrain becoming non-solid after the 30 million mark, this only affected Bets 1.8 to 1.6.4, and stopped being an issue in 1.7.x
• if it refers to chunks not generating after the 30 million mark, this was exclusively an issue in 1.8 and was fixed in 1.9
• if it refers to something else, this has not been stated and should be clarified

Therefore, moving the border out to the proper 30 million point should not cause any issues, and should return these outer chunks to the player's control as they previously were.