There is no functional difference. It’s just a package wrapper version bump, to match the BIP-110 version of Knots. On StartOS 0.3.5.1, the versions numbers had to match in order to “reinstall” from one to the other. This allowed people to switch from the latest version, at that time, of Bitcoin Knots to Bitcoin Core while preserving their Blockchain data.
Since the chain split, none of this is really relevant any more, and you really should update to StartOS 0.4.0.