You can already change an activity to Rucking in Garmin Connect afterwards. If you mean when they are going to add the activity as a native activity on Forerunner 265 itself, the answer is most likely never. Since Forerunner 570 and 970 have already been released, I don't think it's likely any more features will be added to 255/265/955/965.
But based on what I've read, the only difference is that you could set the pack weight on the watch, but doing that isn't affecting any metrics, so setting it afterwards in Garmin Connect works just as well.
