...due to describing features that are already in the game (but done differently by the property).

To clarify: This is not to discredit the property options as good alternatives. It's just that the name can be confusing to some players (if they don't read the Readme), who might think setting them to false disables the Vanilla option whereas in reality, setting them to true enables a new option.

The affected properties:

• renewableIce: Both ice itself (by placing water at a high Y level or in a cold biome and letting it freeze) and dense forms of ice (by crafting them from 4 of the respective less dense form) are renewable. Proposed name: betterRenewableIce
• renewableSand: Sand is technically already renewable through the Wandering Trader. While the option from Carpet Extras is actually viable whereas the Wandering Trader option isn't, it can still be misleading. Proposed name: betterRenewableSand
• renewableWitherSkeletons: Wither Skeletons spawn repeatedly in Nether Fortresses. This case is unique in that the added option isn't significantly more viable than the Vanilla option (although it still feels like a good and fitting idea). Proposed name: electricWitherSkeletons

The problem is that renaming them would mean that any worlds that have them enabled would get them disabled, likely breaking stuff in them.

There's currently no system to automatically migrate rules to new names.

Well, that just means the issue is dependant on establishing such a system (e.g. an API version setup with backwards compatibility mixins) beforehand. Such a system would probably be helpful either way, as being able to rename properties between versions is a major plus in general. Plus, it's not like this issue is very likely to be high priority, so taking your time really is an option.