In a 2019 interview with Deadline, Korsh clarified that his tweet about last-minute changes to the series finale before it aired was just meant as a countermeasure for major spoilers that revealed key plot points in the episode:
“[…] Our promo department made a mistake and they spoiled both Donna and Harvey getting engaged and Donna and Harvey getting married, and one of our extras to some degree spoiled Harvey leaving the firm with the final name of the firm. So, so many things in my mind had been spoiled that I decided to say, I made some big last-minute changes just to throw some doubts into some fans’ minds but I didn’t make any big last-minute changes.”
While fans might have believed Korsh’s deliberate misdirection at the time, the series creator never had any alternative ending in mind even before the leaks, and the broad strokes of the story were pretty much set in stone. Sure, Korsh and his team of writers had to figure out certain story developments to trigger the preordained ending. For instance, during Louis (Rick Hoffman) and Shiela’s (Rachael Harris) wedding, the latter goes into labor and is rushed to the hospital. The emotional nature of this precarious situation spurs Harvey to propose to Donna, as he chooses to leave nothing to fate.
This series of events also allowed Korsh to introduce high stakes before everything was inevitably resolved, with new, bittersweet beginnings on the horizon. In the end, all is well.