You'll be able to literally be as good or better than the fighter in to hit, AC, and damage. Trading 3-4 hit points per level (which becomes less with the false life incantation at 2nd level, and your ability at 10th level) for pretty much all the warlock spells seems "not balanced". This guy makes a better "archer" than the fighter or ranger until level 6 or 8.
Honestly, I balanced this guy against a heavy crossbow fighter. At level 1 (assuming a 17 dex), the heavy crossbow fighter gets 1 attack per round, dealing 1d10+3 damage with a +5 to hit. He's in scale mail, so his AC is 16. Now, he has the option to either get a +2 to hit or +1 to AC, either of which are immensely valueable.
by contrast, the level 1 Celestial warlock (again, assuming a 17 charisma), has one eldritch blast a round, dealing 1d10 damage, and with a +5 to hit. He's wearing scale mail, so his AC (assuming his dex is 14, second highest standard stat score) is going to be 17. Now, he does have the option of using a shield, so that can bump his ac up to 19.
As you go up in levels, the disparity changes a bit, but it's not bad. At level 4, when they get their first stat increase/feat, the crossbow fighter takes crossbow master, the warlock bumps his Cha up 2. Now, (Assuming the warlock took agonizing blast) the warlock is doing 1d10+4 damage per shot, has a +6 to hit, and has that AC of 19. The archer, on the other hand, has the same 1d10+3 damage, the same 16 ac, and the same +7 to hit (assuming he took the archery fighting style), abut also has the option to dual wield hand crossbows for extra attacks, and can attack in melee with his bow.
Overall, it doesnt seem that out of balance to me.
Side note: I'm considering changing the "Change one spells damage to radiant 1/short rest" to something slightly different: "Choose one spell you know that deals damage. Any time you cast that spell, it deals radiant damage instead of its normal damage type. You may change your spell choice after finishing a long rest."
