According to the "Old Worlds Types in Minecraft: Bedrock Edition - FAQ," existing worlds will continue to work. As of the 1.19.20 update, 'old world type' worlds no longer render properly. Instead of rendering chunks according to a player's render distance or the entire world if possible, the game only renders certain chunks. The non-rendered chunks still exist; however, they are invisible. Attempting to travel further will cause the player to be stopped by the same world border used to confine an "old" world.

Steps to Reproduce:

1. Select an existing world save made with the 'old world type.'
2. Select 'Play' to launch the world.

Observed Results:

The world isn't fully rendered with many of the chunks missing. A player cannot pass through areas that were once fully accessible due to the missing chunks messing with the world border.

Expected Results:

The world should render as normal.