Do you have this document?
Or this one?
THE GOOD NEWS IS that Spain has opted for the option that UK nationals will not have to ask for a new residence status as a condition for legal residence. In other words, those eligible for residence rights under the Withdrawal Agreement shall have the right to receive a residence document expressly identifying them as beneficiaries of the withdrawal Agreement.
You may exchange your green residency for the new TIE if you wish. There is no deadline in place yet as the Foreigners Offices workload has been extremely heavy in the last quarter of 2020. The obligation to exchange the old residency for the TIE will come and may be soon. It might be a good idea to submit your application sooner rather than later.
Also, having the TIE in your possession will help you to identify yourself in Spain if you need to and will mean that you won’t need the hassle of carrying your original passport with you.