The most common and most likely answer to this crossword clue is the 6 letter word REBOOK
I've seen this crossword clue in The Guardian
R
E
B
O
O
K
Buddy explains!

The straight part
We know that "to change tickets" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to change tickets = rebook.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 REBOOK mean?
REBOOK verb- To book again.