@Jeff ******
nope you're far from the first person who left the country on a valid extension/re-entry, went and got a DTV then came back in
Most don't bother cancelling their earlier extension
I'm just saying you COULD do that at the immigration office and they would know exactly what you were doing (jumping on the DTV band wagon 😛)
You are correct, even if for some reason you were stamped in on the old extension/re-entry you'd be fine as long as you left before it ran out. Then comin' back just show the DTV print out and you'd get in on it