bastardise
/ˈbɑːstədʌɪz/
verb
Change (something) in such a way as to lower its quality or value, typically by adding new elements.
our biggest fear was they were going to take our script and bastardize it
adulterate,corrupt,contaminate,weaken,dilute,spoil,taint,pollute,foul,defile,debase,degrade,devalue,depreciate,distort