Then you know all that is to be known about what an human is.
You don't need to understand "dolphin-ness" to know that by enabling a dolphin to talk or fly you created a post-dolphin.
Post/Transhumanism is not just about physical changes. Those changes , of course, will create/lead to philosophical/metaphysical questions, but no one understands transhumanism as a "branch of philosophy that questions itself about ...."
A dolphin you enabled to talk is in no meaningfull way a post-dolphin, it is still a dolphin.
The post- prefix presuposes a chronological posteriori point of view, and you can not confirm the veracity of that point of view until you reach it.
In other words you can not call something posthuman until reaching posthumanity and drawing a line, or transhuman until you transcend humanity and can draw the line. The post-adolescent and post-feudalism examples work because they are chronologically in the past and we are arbitrarily drawing a line. Someone in the future might draw the line elsewhere.