It probably does not have enough power, but we can not say it yet.
There are some marginal cases where this feature could be quite useful. For example, if you perform multiclases at a single level of Storm Cleric, the ability to inflict the maximum possible damage to any spell once by brief rest through your Chanel Divinity (up to Charisma fashion times per day) could be pretty attractive But considering all things, this feature probably does not have enough power compared to the Hexblade Curse. First, I will explain why the Hexblade curse is probably more beneficial to a sorcerer, and then I will briefly explain why the overall balance of this characteristic is difficult to judge as written.
Be the best in what you do
An important feature of Hexblade's Curse is that it helps them to be better at what they are designed to do well. A warlock has the best attack cantrip in the game, which invocations can do even better, which attacks several times at higher levels. Hexblades also get the ability to use Charisma to perform melee attacks. All this comes together to mean that the class is likely to attack frequently (at close range or from afar).
In addition to a small healing boost, Hexblade's curse adds a constant amount of damage (which increases as you level up) to each instance of damage to a single target, either by attacks or spells, and doubles your chance of Critical hits in attack rolls That's useful for a class that may be attacking frequently and casting harmful spells from time to time. Its characteristic proposal would be useful to avoid certain resistances or immunities (and the multiclass synergy mentioned above), but not for much more. Add this to the fact that Eldritch Blast does one of the least resisted forms of damage (Strength), and has a feature that is helping this class solve a problem it did not really have.
However, even if this feature is extremely useful in your particular campaign (you may have enemies that are vulnerable to thunder or resistant to necrotic ones), the balance of this feature would still be difficult to calculate. This is because Hexblade & # 39; s Curse is not designed to be a unique feature: it is a part of the class that evolves over time.
What will this do at later levels?
Many of the features of Hexblade are designed to make your Hexblade's Curse stronger at higher levels. Not only does the damage bonus increase with your ability, but the class features of levels 10 and 14 are specifically designed to add features to the Hexblade curse. So, what happens now when the character goes up in level? You will have to propose new features 10 and 14 to let us know. Without those characteristics of levels 10 and 14, we can not judge the overall balance of Master of Elements.
Your function needs a more careful wording
Finally, the Master of the Elements is somewhat difficult to judge because its exact boundaries and its nature are not clear. As written, it allows you to:
change the damage of any spell you cast to cold, thunder, or lightning damage
There are no limits to this change as it is written (only for a damage roll, a round or a throw), which raises several questions. Are you changing the nature of the spell, so this spell will do this new type of damage the next time you cast it? If not, what happens if it is a concentration spell like Hex or Hadar Hunger: the change will last the entire duration of the spell? What if the spell originally caused more than one type of damage: can you change one part of the spell's damage to cold and the other to thunder, or do you have to change the damage of the entire spell to one type?
Depending on the answer to any or all of these questions, the balance of this class function could change. The ability to overcome a resistance or immunity (especially against necrotic damage, which is one of the most common types of damage by sorcerers caused by their spells) could be quite useful. But without clarifying a little the nature of this characteristic, and some more clearly defined ways that this characteristic grows with the character, it is impossible to fully evaluate its equilibrium.