Trying to “understand” what the BSA meant instead of what they said, I think that the intent is that Webelos are all going to wear the tan & green, both at 4th & 5th grade levels once the change becomes official. I think that “this change” is referring to the change from blue as 4th graders to tan & green for everyone will take effect in 2021.
We generally applied the “When your scout outgrows his blue shirt, get him a tan one.” rule at the Webelos level, whether in the 4th or 5th grade age group. It made no sense to any of us to ask a parent to buy a new shirt that would be used for only long enough to sew on the existing badges. If the shirt was $5-$10, and the patches magically self-attached maybe, but definitely not at the going rates for uniform shirts and the actual headaches involved in moving patches to a new shirt. We encouraged everyone to purchase the tan shirts and green pants some time during the 5th grade year, assuming they hadn’t already done so before. That way, the scout could sew his Webelos badge on. We had a couple of families just pull off the old rank badges, and sew the oval on the blue shirt. I let them know technically it was meant to go on the tan shirt, but I wasn’t going to be the “uniform police” about it. Tucked and tidy was my standard of care, not every badge in exactly the right spot. I let parents know something was out of place once or twice to make sure they were aware, then left it alone.