Yes it is possible to decrypt a disk. This option is directly accessible from the
Finder popup menu.
Simply make a ⌃ ctrlclic on the disk icon on the Desktop:
and select the entry starting with
Decrypt…. Then enter the encryption key (or password as you prefer to name it) and confirm.
This function will of course take care of conserving all the data by performing it step by step (because you never have half of a disk free to use for this operation).
Take care the alimentation and the connection cable to your disk are
protected against any break during this operation which may take a few hours.