Buckwheat to WR is an easy transition. Buckwheat has 60 to 90 days of food value. Once it hits the ground, it desiccates quickly and releases the nutrients it scavenged. It germinates quickly and is very competitive with weeds. I tend to be weed tolerant if I have a healthy mix of weeds. Sometimes you get a problematic weed that needs to be addressed. Most summer weeds are done by the time we plant in the fall, but if you have an issue with cool season weeds, spraying becomes more important. You don't need to spray to kill the buckwheat, but you may need to spray to kill weeds. I would simply broadcast the WR into the standing buckwheat. You can then either cultipack (if you have a cultipacker) or mow the buckwheat. With rain in the forecast is best. You can then decide if you want to spray to control weeds. Glyphosate will not hurt the WR seed. Consider mixing a legume with your winter rye. In my area, zone 7a, Crimson clover works well. If you want a short-term clover field the next year, consider medium red clover.
