I'm responding late, but I was reading this because of a Euphorbia of my own which was initially egg-shaped but has grown more ball-shaped. Initially I thought it was because I provided better lighting so it became less-etoliated. But it has also changed colors from a milky blue to a purer green, and offset.
I'm not a Euphorbia expert but from what I've absorbed, there is more natural phenotypical variation in some species and clades of Euphorbia (especially Clade A Rhizanthium) than others, and it's greater than horticulturalists (we) sometimes remember to give credit for because it does not occur across all species of Euphorbia or within a single species. It also occurs differently in cultivation than it would in habitat. This is an important point because it is inconsistent.
For example, sometimes what we call a fertile hybrid of Euphorbia meloformis might just be an unusual variation of the straight species. And what we might think of as a Euphorbia obesa hybrid because it offsets a lot might actually just be the straight species, but an individual plant which is more prone to offsetting (and thus made it a better candidate for horticulture). These kind of variations are more common in some Euphorbias. And because there can be wide phenotypical variation, even when we do
have verifiable hybrids, we can't go by how they look in determining parentage.
Anywya, here is the Euphorbia which made me first start digging for answers:
It used to be more columnar and darker in color, but it has become rounder/fatter and greener. It originally had 5 offsets and now it has 6. lol I'm not sure of the species/hybrid/variety just from looking at it, and with my current understanding I'm not sure I completely trust anyone who says they know what it is.
, if any of this is not helpful feel free to ignore me. Just wanted to add my thoughts and experience. I wish I could find the pictures of when I first got it. I know I took them, but I don't remember where I put them.