For those partnerships that have not elected out of the CPAR regime, amended returns can no longer be filed to report a change to a partnership item starting with the 2018 tax year.
However, the amended return prohibition does not prevent a partnership from filing a superseded original return, which is essentially an amended return filed prior to the return due date (including extensions, but only if an extension was requested).
So you should consider filing an extension for all partnership (and LLC filing as partnership) returns so that you can file a superseded return if a late K-1 arrives or there is another change for 2019 that must be made.
You may file the extension even if you have already filed the return. You would want to alert K-1 recipients that there may be a corrected K-1 so they can extend their own returns.
Be sure to file the superseded return on or before the extended due date of the return.
The IRS is continuing to update forms and procedures for amendments or administrative adjustments for partnership returns.