By the described rules? No. It's because it isn't actually a spell slot, it's a class feature that only lets you use the selected appropriate level warlock spell.
Not RAW but it doesn't seem like an overly game breaking house rule. If one of my players had a Fiend Warlock and really wanted to lean into the summoning of fiends aspect of it I would allow them to cast a higher level Infernal Calling for instance with their Mystic Arcanem. Similar if they wanted a higher level Fey with Conjure Fey for instance.
Can you choose a lower level spell as your arcanum and cast it at the higher level?
If I was the DM and you asked, I’d house rule that you could do it. But RAW the answer is no.
Professional computer geek
But, what you can’t do and what I wouldn’t let you do either is change your Mystic Arcanum. Once you pick it you’re stuck with it forever.
Professional computer geek
By the described rules? No. It's because it isn't actually a spell slot, it's a class feature that only lets you use the selected appropriate level warlock spell.
Also known as CrafterB and DankMemer.
Here, have some homebrew classes! Subclasses to? Why not races. Feats, feats as well. I have a lot of magic items. Lastly I got monsters, fun, fun times.
Not RAW but it doesn't seem like an overly game breaking house rule. If one of my players had a Fiend Warlock and really wanted to lean into the summoning of fiends aspect of it I would allow them to cast a higher level Infernal Calling for instance with their Mystic Arcanem. Similar if they wanted a higher level Fey with Conjure Fey for instance.
Yeah that's no fun. My table homebrewed that you can switch you MA spells at level up similar to your Eldritch Invocations.
Thanks for all of the advice! I am still a low level warlock but will talk to my GM