Can I decrypt a tablespace that has previously been encrypted? Can I disable TDE?

Question: Can I decrypt a tablespace that has previously been encrypted? Can I disable TDE?

Answer:
In versions prior to 12.2, you cannot.
From Version 12.2, you can.

In Oracle Versions prior to 12.2, this is what Oracle writes about decrypting a tablespace?

"You cannot decrypt a tablespace that has been created encrypted. You must create an unencrypted tablespace and re-create the database objects in the unencrypted tablespace."

In version 12.2, Oracle has added support to decrypt both online and offline tablespaces.

The syntax for an offline tablespace decryption would be

administer key management set keystore open identified by software_keystore_password; 
alter tablespace TEST_TABSPC offline;
alter tablespace TEST_TABSPC encryption offline decrypt;
alter tablespace TEST_TABSPC online;

An online decryption would executed like this:

alter tablespace TEST_TABSPC encryption online decrypt file_name_convert = ('test_tabspc.dbf', 'test_tabspc_decrypted.dbf');

For the online method to work, compatible should be set to 12.2, and there must be enough storage available for the database server that you can hold a copy of the entire tablespace to be decrypted.