Conjure Minor Elementals (Basic Rules) and similar spells have the following text at the end of the spell block. My D&d Beyond account says it comes from Basic Rules, pg. 226, and AFAIK there's not any updated versions.
At Higher Levels. When you cast this spell using certain higher-level spell slots [...] more creatures appear: twice as many with a 6th-level slot and three times as many with an 8th-level slot.
Following a very literal reading, there seems to be a weird edge case: a 7th (or 9th) level slot is not one of the "certain higher-level spell slots" listed in the spell description, and thus would have no benefit vs. casting it at the lowest possible level.
This is extremely surprising to me, because I can't think of any other situations where a higher level spell slot (7) produces a weaker effect compared to a lower level spell slot (6) when casting the same spell. I have a feeling this can't be intended, but I can't find any official rules or rulings that would support the idea that casting using a 7th level slot should allow the Druid to summon additional creatures as if he used a 6th level slot. I have carefully reviewed the rules for Spellcasting including Spell Slots and Casting a Spell at a Higher Level to see if a general rule might allow a higher level slot to replicate the effect of a lower level slot, as well as looked for specific errata or Sage Advice for this Spell and come up blank.
AL DMs also have less leeway than most DMs for adjudicating situations according to "Rule of Cool" or "Rules as Intended". The D&D Adventers League Forgotten Realms DM's Guide says
You can issue rulings to your table when the rules of the game are ambiguous or vague, but you must otherwise adhere to the rules as they are provided in the core rulebooks and can’t change them or make up your own; “house-rules” aren’t permitted for use.
While they aren’t official rules that must be followed, the Sage Advice column, tweets from the D&D Team on Twitter, or even discussions with other DMs on your favorite social media platform can provide good insight on how others adjudicated a particular issue
As I'm planning to run a druid in AL for the first time starting soon, I'm interested in knowing if I have to make sure to keep exactly a 6th or 8th level slot available to gain benefits from casting these spells at higher levels, or if 7th and 9th level slots would work as well.
I don't think any official rulings exist, so I'm mostly hoping to get an answer that is based on experience in AL play, but in case I missed something, I prefer answers higher on this priority list to lower:
- Answers such as "Yes, in AL play you can gain the 6th level effect when using a 7th level slot" (or "No You can't [...]") because of a general rule I missed that covers this specific case.
- "Yes, you can [...]" or "No you can't [...]" because an updated version of the spell or general spell casting rules was published in [Name of Source Book], Errata, or the D&D Adventurer's League FAQ.
- Answers such as "Yes, you can [...]" or "No you can't [...]" because the situation was clarified in Sage Advice.
- Answers such as "Yes, you can [...]" or "No you can't [...]" based on tweets from the D&D Team on Twitter.
- Answers such as "Yes, you can [...]" or "No, you can't [...]" based on direct experience in AL play with how DMs have adjudicated this specific spell, or other spells following the same pattern (conjure woodland beings, etc.)
- Answers such as "Yes, you probably can [...]" or "No, you probably can't [...]" based on general observed trends in AL play with how literally DMs in AL play interpret rules vs. acting as a "Facilitator of Fun" or trying to apply RAI.