can we change characterset and tablespace block size after database creation?

If Tablespace block size is showing 4096.It is because while creating the database we had given db_block_size=4096,so after creation of database we cannot able to change the block size to 8192.
And also we cannot able to change the change the characterset after the database creation.

If you want to find the block size of tablespaces?

select block_size from dba_tablespaces;

If you want to find the characterset of the database?

select * from NLS_DATABASE_PARAMETERS;

select * from NLS_DATABASE_PARAMETERS where parameter='NLS_CHARACTERSET';