As long as you are taking the ATP exam at a place that had an established 121 or part 135 training program then the written does not expire. I have heard this out of he FsDo my self. However I have heard dif answer from people. I think it's a grey area so you end up hearing dif responses from dif people.
But yes I got my ATP well after my written was expired and not from my 121 airline.
It is not a grey area. It says right in the regs you may use an expired written if you are employed by a 121 at the time of the practical test and have completed that companies upgrade training.