No they are not backward compatible.
It was not just the part of the hash that is being checked (the leading zeroes counted), but also the hashing done itself. They would actually do a keccak256 hashing twice (whisper v5 leftover). If you’re lucky to have enough leading zeroes also with the other calculation, your message will not be dropped.
If you want to remain compatible you have to either lower the PoW requirement even more (practically 0) or (temporarily) implement and use both calculations at receiving end.