Billy:
Our son is three months old now. We change him in the Living room as much as we do in the bedroom, probably even more so. what we use is a portable changing table that happened to come with the playpen. It will sit on both the crib and the playpen. It has a short strap that you can fasten around his waist to prevent falls. Its also sort of u-shaped.
My input is this: a better thing might be a changing caddy, with space for diapers, lotion, wipes and burp cloths- and a spot where you can place the dirty diaper before you toss it. I can take pictures in the morning, but I guess I'm just trying to tell you that it might not be necessary to reinvent the wheel. Portable seems to be king these days.
For my older daughters we had a dedicated changing area and I remember using it. But what we have now is so much easier. Its light, and goes with us everywhere. If we are in the bedroom, we use it there, and in the Living room, we set it on the playpen (it has the same width as the crib) and use it there.
I really like the convertible crib idea though. I have used them in the past. Worked really well. And if POP-POP made it... Well it might just last generations!
Glen