Sudden change for American humorist into small red jumper (10)
I believe the answer is:
Here is my best explanation:
'sudden change for american' is the definition.
(informal US term for a sudden change)
'humorist into small red jumper' is the subsidiary indication.
'humorist' becomes 'wit' (wit is a kind of humorist).
'into' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'red' becomes 'che' (Che Guevara the Marxist revolutionary).
'jumper' becomes 'roo' (I've seen this in other clues).
'wit' put into 'sche' is 'switche'.
(Another definition for switcheroo that I've seen is "Informal US variation".)