The new /execute syntax is made mandatory in 1.19.50. This is a breaking change, which will cause many existing maps (specifically, command blocks) and add-ons to stop working. Migrating to the new syntax might be infeasible for creators who have already used many command blocks with /execute.

My suggestion is to have the game automatically update all command blocks in a world upon loading an older version world with 1.19.50. Similar to the 1.17 -> 1.18 (caves and cliffs) update, players will be prompted to make a copy of the old world. The new /execute syntax contains all the functionality of the previous one. I propose the following way to convert between them.

1. Old - /execute <target> <position: x y z> <command>

New - /execute as <target> at <target> positioned <position: x y z> run <command>

2. Old - /execute <target> <position: x y z> detect <position: x y z> <block> <data> <command>

New - /execute as <target> at <target> positioned <position: x y z> if block <position: x y z> <block> <data> run <command>

If my understanding of the new syntax is correct, the old and new commands I proposed should behave exactly the same way in all situations. Players can then use the new syntax for future command blocks. An easy implementation for this conversion is to use Regex (regular expressions), a reliable and widely available tool for string manipulations.