I have a PHP DateTime object with microseconds created as follows:

$time = microtime(true);
$microseconds = sprintf('%06d', ($time - floor($time)) * 1000000);

$dt = new DateTime(date('Y-m-d H:i:s.' . $microseconds, $time));

How can I modify the microseconds value of $dt, without creating a completely new DateTime instance?

You can't.

There are three methods that can modify the value of a DateTime instance: add, sub and modify. We can rule out add and sub immediately because they work in terms of a DateInterval which does not have sub-second precision.

modify accepts a string in one of the standard recognized formats. Of those formats, only the relative ones are of interest here because the other ones work in an absolute manner; and there is no relative format that allows tweaking the msec part (that unit is not recognized).

http://stackoverflow.com/questions/21756701/modify-microseconds-of-a-php-datetime-object