Grapefruit comes from the hybridization of C. maxima with an orange, but my question was what would happen if one of the parents was changed?
Grapefruit is a specific cross that has a specific origin.
There are several traditional Japanese varieties where C. maxima was introduced into mandarin (C. reticulata) breeding. (Think for example about the slightly sour more aromatic flavor of Satsuma mandarins)
The result will be a little bit different from the typical grapefruit flavor.
These fruits will be, in many ways, analogous to grapefruit, but they will not truly be real grapefruits. They will be similar but they will not have quite the exact same type of flavor.
This can be difficult to describe.
I can get into the chemical components of the aroma, if you want. One of the most important aroma compounds in both pomelo and grapefruit is nootkatone. It occurs only at very low levels but it's very potent at those low levels. It smells a little in the direction of mouth-puckering rhubarb but also a bit of a cleaner green and woody dimension.
But in grapefruit, in addition there is 1-p-Menthene-8-thiol (also called "Grapefruit mercaptan"), which smells kind of distinctively like "grapefruit" but in a completely different way. It was a subtly sweet aroma that is slightly pungent (slightly mild smell, in a way, yet "potent" in feeling at the same time). And strangely, if you compare it to the smell of coffee there is some subtle underlying similarity in the feel. It brings to mind more of a "pink grapefruit" feel. This almost feels more like edible grapefruit candy than natural grapefruit.
The distinctive aroma of orange (and especially Valencia variety of orange) comes from a substance called valencene, which is actually related to nootkatone, but doesn't have the oxygen atom that nootkatone has. They do kind of share some similarity in overall underlying feel but are also very different. Valencene does not have the rich mouth-puckering slightly "dirty" rhubarb feel that nootkatone has.
I think another important compound you'll find in many of the more aromatic mandarins is sinensal. It has a "bright" and "juicy" feeling, but also some "orange-citrus" tonalities like D-limonene does. Again, it's very reminiscent of "orange" but in more of a different way from valencene.
This is only a basic overview only touching the surface. Of course the full story is much more complicated.
When you cross a pomelo with a mandarin, you're going to get something that's a little bit similar to tangelo in flavor. But of course it will be more sour yellow and aromatic because it will have a higher percentage of C. maxima ancestry than a tangelo does. So maybe somewhere between a tangelo and a "grapefruit" sort of feel, but not with all of the distinctive aroma of a real grapefruit.
Keep in mind your offspring will likely be tasty but will contain a huge number of seeds.
The C. maxima x C. reticulata hybrids tend to have a little more hardiness than regular sweet oranges, so should grow well (or at least acceptably) in climate zone 9b.