The 5th level spell Conjure Elemental spell states (PHB, p. 225, bold added):
An elemental of challenge rating 5 or lower appropriate to the area you chose appears in an unoccupied space within 10 feet of it...
The DM has the elemental’s statistics.
So far, that's certainly true. The four main elementals in the Monster Manual are all CR 5 creatures, so the stats are certainly available. And as has been mentioned before, the "or lower" part isn't likely to come up (and will only be an issue for a DM if the DM wants it to). However, there is one additional rule on upcasting that gave me some pause:
At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the challenge rating increases by 1 for each slot level above 5th.
Now I'm concerned because there are few published stats of Elementals (I'm aware of) of Challenge Rating (CR) 6, 8, or 9. Sdjz pointed out this list showing some that exist (and plenty of CR-7), but there are still several omissions (e.g., a fire elemental of CR 6, or any elementals of CR 8). I realized I often wouldn't be prepared to provide elementals with statistics that could be summoned by an upcast version of this spell.
The DMG gives suggestions for altering existing creatures (p. 273), but nothing about deliberately raising its CR. Of course, there are guidelines in the DMG for how powerful creatures of various CR should be (especially on page 274). But I'm hesitant to simply give them all the average recommended stats since the existing standard CR-5 elementals vary wildly in how they go about fitting (or violating) these guidelines to begin with, and they vary quite a bit in statistics from each other (both in simple stats like HP and AC, and in complicated ones like special means of movement or unique actions).
This made me realize that I didn't have any clear sense of how to adjust their existing statistics to fit with an upcast summoning. Especially since, as a DM, I won't always know what level a PC might cast this spell at on any given day.
If I'm running a group containing a spellcaster with access to 9th level spells who can cast Conjure Elemental, is there an accurate, fair, and ideally fast way of adjusting the Elemental's existing statistics to make them higher CR creatures? Or will I just have to speculatively meticulously create CR 6, 8, and 9 elementals in case it ever comes up?