Grint, now 37, began playing Ron at age 12 and finished at 22, acknowledging the role profoundly changed his life.
He has written a letter to Alastair Stout, the child actor cast as Ron Weasley in the forthcoming HBO Max TV series adaptation.
The new "decades-long" TV series will adapt JK Rowling's books, with each season based on one of the seven novels.
Production for the series began in July at Warner Bros Studios Leavesden in the UK, featuring a new cast including Dominic McLaughlin as Harry and Arabella Stanton as Hermione, with a debut expected in 2027.