I'm in almost the exactly the same situation (Gold to Platinum), except my qualification period ends in April.

From what I gather, if you simply credit your flight in April, it will still count for March and your Platinum will be valid through March 2027. The only question I couldn't answer is what happens if you don't credit the March flight, then credit another flight taken in April, bumping yourself to Platinum until April 2027 and only then request credit for the March flight. I'd expect them to not deduct a month from the status you already earned until 04/27, but whether they'd add XP to the the 2027/28 counter is questionable.

No data points from me though, I decided to play it safe and credited a flight to SAS instead to avoid upgrading FB status too early. Part of the motivation to do it was that you can't really use some benefits (e.g. baggage) without having FFN attached to the booking, so if I wanted to employ the strategy above, I'd have to pay for my bag which would cost more than the value of status for month.