How easy is using 16v head on 8v cars ?

[VWMAN]

For example guys, if i had mk2 8v GTI (digifant), or i got mk3 8v GTi (simos) could i use 16v kr/pl/9a/6a/abf head easliy on mi 8v GTi ? or i got mk3 8v GTi (simos)

What else should i change apart from heads.??

thanks...