Downstile.dyalog

 Downstile←{ ⍝ ⌊ which allows lowercasing letters
     0≠⎕NC'⍺':⍺⌊⍵ ⍝ dyadic case
     2|⎕DR∊⍵:⌊⍵   ⍝ shortcut if all numeric
     w⊣(∊w)←{
         2|⎕DR ⍵:⌊⍵ ⍝ numeric
         11::⍵      ⍝ fallback to no-op
         819⌶⍵      ⍝ lowercase
     }¨∊w←⍵
 }