What are the ramifications of this change to upcasting spells?

Question

I was looking at the Evocation Wizard's Overchannel feature which states:

When you cast a wizard spell of 1st through 5th-level that deals damage, you can deal maximum damage with that spell.

This requires that you use a spell slot of 1st through 5th level, unfortunately a Wizard has at most three 5th level spell slots so I came up with the idea of "variable upcasting".

An example of "variable upcasting":

• If a spell is a third level spell and you want to use a sixth level spell slot to cast it, normally this would make the spell a sixth level spell. This change would allow you to upcast the spell as if the spell slot were other levels.
• The minimum level would be the regular level of the spell.
• The maximum level would be the regular level of the spell slot.
• You could cast the spell at any level between these numbers.

Regardless of what level you actually cast the spell at the spell slot you initially chose to use is expended in its entirety.

Note: this option would be available to all casters, not just wizards and, perhaps to avoid being too broad, the option does not have any effect on cantrips.

Thanks to user @MarkWells for describing my houserule in a much simpler way:

You can use a spell slot as though it's a spell slot of a lower level

What are the balance implications of allowing this "variable upcasting" option?

Answer 1

Balanced, only a few interactions will be impacted by this.

What you are proposing is a slight modification to the rules for casting at a higher level. RAW it reads (emphasis mine):

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level.

What you are proposing is making the bolded portion of that quote optional. There are no rules for this because it very rarely beneficial, except in particular circumstances such as Overchannel. The Twinned Spell metamagic will also benefit from this at it costs a number of sorcery points equal to the slot level.

Effectively you are turning a higher level slot into a lower one, this should (almost) always be a net loss and should be balanced. In fact there are already two RAW methods for doing this, the Sorcerer's Flexible Casting feature and the Variant: Spell Points rule from the DMG. Both of them have the added benefit of giving you the extra magic of the higher slot as resources you can use later.

The likelihood of this rule coming into play is actually fairly low, in my experience players are far more likely to run out of higher slots than lower ones. The chance of you having a higher slot to variably upcast with and not a spell or that level that would be more useful it fairly low.

Your proposal gives some added flexibility which is the same benefit as the Variant: Spell points but with the added cost of reduced damage, otherwise it does not allow you to do anything you couldn't already do with full resources. Additionally, your spells will be easier to counterspell or dispel.

The cost of these changes if equivalent to the benefit. This seems to be a balanced house rule, enjoy playtesting.