Change positions in bottom class (9)
I believe the answer is:
rearrange
'change positions' is the definition.
'bottom class' is the wordplay.
'bottom' becomes 'rear' (synonyms).
'class' becomes 'range' (both can mean category).
'rear'+'range'='REARRANGE'
'in' acts as a link.
(Other definitions for rearrange that I've seen before include "Move to new positions" , "Alter" , "What 8 [ADJUSTER] may do" , "Organise another way" , "Put in new order" .)