EXtending the knowledge of long addition to digits on paper may happen immediately or take few more lessons. The children are motivated. In writing, long addition is hard to learn, but easy to perform.
Like counting words, digits work if we know them and their order. Nothing in 7 reminds seven, but we can count through 0, 1, 2, 3, 4, 5, 6, 7 and tell the result. To keep it, one token (a digit) is enough.
Remember how we navigated the board? Now is the time to to make and use charts of addition and practice adding very big numbers instead of single digits.