upgrade from 10.2.0.4 to 12.1.0.2

Hi,

I'm planning to upgrade oracle database from 10.2.0.4 to 12.1.0.2

along with character set conversion

FROM

NLS_CHARACTERSET               WE8MSWIN1252

NLS_NCHAR_CHARACTERSET         AL16UTF16

TO

NLS_NCHAR_CHARACTERSET         UTF8

NLS_CHARACTERSET               AL32UTF8

after 1st attempt schema import I see( from import log file )  about more than 100 tables and all together  700 column length to be changed   ,

I can do that manually , is there any efficient method to handle this ?

I'm not sure how to handle this upgrade ,   is this correct approach ?  

I need some guidance on this .......

Thanks in advance.

TCY