In Oracle 12c you should use the Database Migration Assistant for Unicode (DMU) to perform this operation.
Your proposed conversion is very straightforward and shouldn't be a problem, however: Are you sure this is what you want to do? Oracle recommends AL32UTF8 above UTF-8, unless it is absolutely neccessary:
Oracle supports two encodings of Unicode as the database character
set: UTF-8 through the AL32UTF8 character set and CESU-8 through the
UTF8 character set. (Note the use of the hyphen in the name of Unicode
encoding and the lack of a hyphen in the name of Oracle character set.
This differentiation is used throughout Oracle documentation.) UTF-8
is a multibyte varying width Unicode encoding using 1 to 4 bytes per
character. CESU-8 is a compatibility-only encoding, discouraged for
information exchange by the Unicode standard. CESU-8 is very similar
to UTF-8 except that so-called supplementary characters encoded by
UTF-8 in 4 bytes, CESU-8 encodes as pairs of 3-byte codes. Oracle
Database has deprecated the UTF8 character set, so it should not be
used as a database character set except when explicitly required by an
application, such as Oracle E-Business Suite Release 11i.