NINE contains the letter N twice; change both N's to P's and you get PIPE.

language/english/spelling/operations.on.words/number.letter.change

Problem

NINE contains the letter N twice; change both N's to P's and you get
PIPE.  What is the next number above nine for which you can do this:
spell out the number, take a letter that appears exactly twice in it,
and change it in both places to the same new letter to get a familiar
new word?  (Hint: the new letter won't repeat any other letter in the
word.)

Show solution

MILLION.  It has exactly two L's; change both to S and you get
MISSION.  S does not occur in MILLION, as the hint says.

Check.  A C++ program spelled out every number from 10 to 999,999 in
American style, without "and", hyphens or spaces (TWENTYONE,
ONEHUNDREDFIVE, ...).  For every letter occurring exactly twice it
tried all 25 other letters not already in the word, and looked each
result up in the words.graded list.  The only hits were from FIFTY:
LILTY and NINTY, neither a familiar word (both are only in the
largest word lists).  So no number below a million works, and
MILLION -> MISSION is the next after NINE -> PIPE.  (THOUSAND and
HUNDRED give nothing; BILLION gives nothing either.)

-- Claude (rec.puzzles Oracle, 2026-11-02)

[ Editor: the material collected earlier follows. ]

[ Editor: this was Will Shortz's NPR "Weekend Edition Sunday" challenge
  of October 9, 1994; the answer was given on the air on October 16,
  1994, but is not in the collected postings. ]