I wanted to pick your DM brains about allowing a Cleric of the Tempest domain to swap one of their domain spells.
One of the players in our campaign would like to swap Control Water (PHB, p.227) for Storm Sphere (XGtE, p.166). It seems to be in line with the whole flavour of a Tempest Cleric and they are both 4th-level spells.
Note: Storm Sphere appears in Xanathar’s Guide to Everything, so it had not been published at the time when the Tempest domain was published.
Would it be unbalanced to allow this spell swap for the Tempest cleric’s domain spells?
I would really appreciate any answers that draw on own experience of allowing tweaks for a Cleric domain spells.