We got ours done about a month ago. You have to call and schedule an appointment. It is best to call either first thing in the morning, or an hour or less before close. Be ready to keep dialing until you get through. It took us 3 days of trying but we finally got through. 
We had an appointment set for 10:30 in the Porto office. We showed up at 9:45, took a number, but were only seen at noon! The appointment only gets you through the door I guess! Once there, they asked if we wanted to keep our card and have our address updated in the system, which would be free, or get new cards with the new address, which we would have to pay for. We opted for the latter and received our new cards in the mail about a month later. The added benefit of getting a new card is that they added a year to the validity of our residence, so went from two to three years.  The cost was 36€ for the new card. They let you keep your old card so you don’t have to walk around with no identification. One caveat, if it’s important to you, is that the new card comes with a new number.

I hope this helps.

@bman63 Are you saying that by changing your address your resident permit is automatically extended by an additional one year? Thanks for the update on the new number on the new card. I recently ran into that on my US passport renewal and had to update my Portugal bank with the new number and validity period. Anyone with the old resident permit number would need to be updated.

@jonesdn2020 I wish I could say it with certainty! All I can say is that it happened to both me and my wife. I know how different people experience different outcomes with bureaucracy, but it was a nice benefit we weren’t counting on!

Please can someone say what documents we need to change address on the Financas web site for our NIF?

I attached our lease and received an email advising the following,

In addition to the rental contract, a document expressing interest or request issued by the uoi registration platform at SEF must be sent stating the new address

Thank you for assist.

We just uploaded our resident permits to change the address from the US to our PT address on the permits.

Have you got the link to where you did it please? Thanks!

We followed this guide and customized where necessary for our fiscal rep since it wasn’t them. How to update your Portugal NIF address on Portal das Finanças (bordr.com)