My son, also starting K next year, started preschool this past fall. We had the same issue, more or less, ours was more a varying bed time anywhere from 8-9:30. We just started gradual, and always set the time on the stove starting at 30 minutes before bed time. We would set it for 15 minutes to mark the "Half way" Some nights he would go at the halfway point to do his teeth etc, othertimes he would wait until the end which we allowed for so when he did it at the half way, he just went down a few minutes early. He can't tell time yet so it all worked out :)
Although he was in bed by 8pm with in about two weeks, it did take a little time for him to fall asleep that early. If there were nights he was having a hard time, he could lay in bed and look at a book, quietly, and he had to stay in bed. If he started getting up and down to play the 'I need...." game then the book went on the shelf and he just had to lay there.
It wasn't/isn't foolproof, there are still nights we slack and he gets to bed later, but it only takes once of having to wake him up in the morning to get ready for school to remind me that I need to get him to bed so he gets enough sleep! We found the transition to be easier than we expected and actually realized that he really needed the earlier bedtime, we just weren't noticing the signs. We have always put the twins to bed at teh same time, ever since they were infants, and then he got to stay up a bit later to get some 1:1 time with us. Now everyone is in bed at or near the same time.
Good luck!