Jelly,  28 27  26 bytes

-1 thanks to Erik the Outgolfer & dylnan

^{Not a very Jelly-friendly challenge!}

ḟŒs$Ƈ<”[s5Ḅ+97Ọż@ṣ”_$FṁLŒl

A monadic link accepting and returning lists of characters.

Try it online!

How?

ḟŒs$Ƈ<”[s5Ḅ+97Ọż@ṣ”_$FṁLŒl - Link: list of characters    e.g. "MfUNE_?"  (shorthand for ['M','f','U','N','E','_','?'])
    Ƈ                      - filter keep if:
   $                       -   last two links as a monad:
 Œs                        -     swap-case
ḟ                          -     filter discard
                           - ...i.e. keep A-Z,a-z since they change when the case is swapped
                           -                                  "MfUNE"
      ”[                   - literal character                '['
     <                     - less than? (i.e. is upper-case?) [1,0,1,1,1]
        s5                 - split into fives                 [[1,0,1,1,1]]
          Ḅ                - from base two (vectorises)       [[23]]
           +97             - add (vectorises) ninety-seven    [[120]]
              Ọ            - from ordinals (vectorises)       [['x']]
                    $      - last two links as a monad:
                  ”_       -   literal character              '_'
                 ṣ         -   split at                       [['M','f','U','N','E'],['?']]
               ż@          - swapped @rgument zip             [[['M','f','U','N','E'],'x'],['?']]
                     F     - flatten                          "MfUNEx?"
                       L   - length (of input)                7
                      ṁ    - mould like                       "MfUNEx?"
                           - ...removes any excess characters
                        Œl - lower-case                       "mfunex?"