Tuning down a guitar’s strings a whole step, from standard EADGBE to DGCFAD, typically does not pose a significant risk of breaking the guitar neck. However, sudden extreme changes in tension can cause minor adjustments in the guitar’s setup, such as truss rod tension and bridge adjustments. These changes might slightly affect the guitar’s playability and intonation. To minimize potential issues, it’s advisable to make gradual adjustments to the tuning, allowing the guitar to acclimate to the new tension. Additionally, using appropriate string gauges and consulting a professional for setup adjustments can help maintain the guitar’s structural integrity while tuning down.

I have never heard or seen a guitar neck break from tuning down the guitar’s strings a whole step. I remove the stings completely when changing the strings on my guitar and nothing happens. Removing the strings, in my opinion, is the ultimate string loosening.