The most common and most likely answer to this crossword clue is the 14 letter word TRANSMOGRIFIED
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result TRANSMOGRIFIED
Buddy explains!

The straight part
We know that "Completely changed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case completely changed = transmogrified.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TRANSMOGRIFIED mean?
TRANSMOGRIFIED adjective- Altered, transformed, or mutated into a form that is grotesque or amusing.
- To completely alter the form of.
- To completely alter one's form.