Change expiry date of TDE certificate of User Database without changing Thumbprint
Referencing the information in Updating an expired SQL Server TDE certificate, the post explains that it is easy to create a new certificate with an updated expiration date for use in TDE.

USE [master]
GO

CREATE CERTIFICATE NewTDECert
WITH SUBJECT = 'New TDE DEK Certificate',
EXPIRY_DATE = '20181231';
GO 

USE [YourDatabase]
GO
ALTER DATABASE ENCRYPTION KEY
ENCRYPTION BY SERVER CERTIFICATE NewTDECert;
GO


After executing the above commands, your database now uses the new certificate with regards to TDE.

However - the post makes it very clear that you still need to keep the old certificate in case you want to restore the database from a backup created when the old certificate was being used.  

From that post (highlighting mine):


  It is up to you whether you want to drop the old certificate from the
  SQL Server instance. You should always keep a backup of the old
  certificate in case you need to restore a TDE enabled database using
  an older backup that used the old key.


Additonally (from that post), 


  It is paramount to backup the TDE certificate after any certificate
  changes as this is required to restore the database to another SQL
  Server instance. We can issue a backup certificate command for the new
  certificate as shown below.


USE [master]
GO

BACKUP CERTIFICATE NewTDECert
TO FILE = '\\SQLP2\temp\NewTDECert.cer'
WITH PRIVATE KEY (FILE = '\\SQLP2\temp\NewTDECert.pvk',
ENCRYPTION BY PASSWORD = 'str0ngPa$$w0rd');
GO 




Additional information can be found in Replacing an expiring SQL Server encryption key.  (Highlighting mine).


  To rotate the certificate for TDE, add the new certificate as above,
  then execute the command ALTER DATABASE ENCRYPTION KEY with the
  ENCRYPTION BY SERVER CERTIFICATE clause:


ALTER DATABASE ENCRYPTION KEY
    ENCRYPTION BY SERVER CERTIFICATE newCertificate;



  SQL Server re-encrypts the database encryption key with the new
  certificate, and drops the encryption by the old certificate when it’s
  finished. And as before, the data itself isn’t re-encrypted, so the
  process finishes almost immediately.
  
  One caution: Always keep at least one backup copy of every certificate
  you use. If you ever need to restore a database that used encryption,
  you’ll need the certificate that was in effect at the time the backup
  was created. Make a habit of creating a certificate backup immediately
  after creating it in SQL Server. Store the backup in a safe place;
  also keep a copy of the passphrase you use to encrypt the certificate
  backup, preferably in a different safe place for security. Retain
  these forever, or until the last database backup that may possibly use
  them has been purged.