A study by George Borjas (Harvard) and Kirk Doran (Notre Dame) indicates that, compared to mere contenders for the prestigious prize, winners of the Fields Medal become less productive but more willing to take intellectual risks.
The two effects in fact seem to be related: the researchers attribute about half of the drop in the number of papers published annually after mathematicians win the Fields Medal to their increased willingness to venture outside their mathematical comfort zone into subfields less familiar to them.
This "cognitive mobility" exhibited by Fields medalists does have an upside, as Business Insider reports:
In the late 20th century, there are three great examples of mathematicians who made great contributions in one area of mathematics, then went on to make huge contributions in a distant subject years later. All three were Fields Medal winners.