I have a client whose spouse is a permanent US resident (from the islands originally) and was recently issued a permanent resident ID & card.  They were married in 2017 but due to not having any identification information and the client still working through the residency status, we went ahead and filed the return in 2017 as single.  Now to the 2018 return, the client spouse was recently issued the permanent resident card stating residency since February 2019.  As they were considered legally married in 2017 and with the process now complete in terms of getting the residency identifier.  I'm thinking the client can amend the prior year return to claim the married filing jointly status as well as file the 2018 return as jointly.

They are working on obtaining SSN number, however as this may take some time.  Would there be a right way to handle this process or the steps in the process.  My research indicated applying for an ITIN (form W7 - attach to tax return).  The application states not to use if you  have or are eligible to receive an SSN.

Also would it be best to amend the prior year 2017 return (to married status) prior to filing the 2018 return?