I tend to think of "downconvert" to mean less output "resolution" (pixels per frame), not khz signal differences.
For instance when you go from 720p to 1080i you have to
add pixels to the resultant 1080i frames so "PIXEL REPLICATION" or some sort of "interpolation" is being done to calculate what extra pixels to add. I call that "UPCONVERT".
When you go from 1080i to 720p you have to remove pixels to fit into the smaller resolution frames. "PIXEL DECIMATION" is done to elimate pixels which cannot be displayed at the lower resolution. I call that "DOWNCONVERT".
Doing a websearch on "720p to 1080i" I find the term "upconvert" is used many times (on different sites), but "downconvert" is generally only used when talking about 1080i->480i, 720p-> 480i.
I found one "DTV glossary" that attempts to explain this.
They seem to acknowledge that "upconvert 720p to 1080i" is the common usage, but they consider it a "misnomer":http://www.nortek.net/learning_center/dtv_glossary.htm
A term used to describe the format conversion from a higher resolution input signal number to a
lower display number, such as 1080i input to 480i display.
The term used to describe the conversion of a lower apparent resolution to a higher number, such
as "upconverting" 720p to 1080i. This is a misnomer, though, since to accomplish this, the
horizontal scanning frequency is actually lowered from 45kHz to 33.75kHz. Resolution quality is
not improved by this method.
So - even though it seems that "upconvert 720p to 1080i" is the common usage, I vote that we come up with the new term "crossconvert" when going between formats where the higher pixel resolution format (e.g.: 1080i 1920x1080 vs 720p 1280x720) has a lower horizontal scanning frequency (e.g.: 1080i 33.75khz vs 720p 45khz).
Upconvert 480i to 1080i
Crossconvert 720p to 1080i
Downconvert 720p to 480p
If people don't know what I mean when I say "crossconvert" then I will stick with "upconvert 720p to 1080i" because I value final pixel resolution more than horizontal scanning frequency.