What is it called phonetically, when Americans change the pronunciation of "pronunciation" to "pro-*noun*-ciation"?
This is not a phonologically-defined vowel change, unless you have a list of words with a similar phonological pattern.
Instead, what you are doing with pro-nounc-iation is maintaining the structure of the stem, pronounce, instead of allowing it to undergo a vowel reduction ([aʊ] -> [ʌ]). (Incidentally, the vowel reduction occurs in the first place because that syllable, which is stressed in the verb pronounce becomes unstressed in the word pronunciation; of course, we aren't recreating this reduction every time we pronounce the word — the reduction became "set" in that word in the lexicon at some point in time.)
In any case, keeping the stem constant as you are doing could be considered a form of leveling. That is, you are regularizing the stem analogically, thus keeping the stem constant across different derivational forms.
I believe this is called analogy: because a similar word in similar circumstances is pronounced with ou, some speakers will pronounce the u in pronunciation likewise, and its sound may eventually change completely. See Wikipedia on analogy in linguistics. Variation (often by contraction) and analogy are the great opposing forces that continually mould our language into new forms.
Depending on the scale of the change, it could be considered a vowel shift:
A vowel shift is a systematic sound change in the pronunciation of the vowel sounds of a language.
The specific vowel shift that hits home for me is the Northern cities vowel shift:
- Raising and tensing of /æ/
- Fronting of /ɑ/
- Lowering of /ɔ/
- Backing and lowering of /ɛ/
- Backing of /ʌ/
- Lowering and backing of /ɪ/
Notice that there are specific terms used to describe the individual changes. Moving from "nun" to "noun" would probably line up with this description and therefore receive the term "backing":
Backing of /ʌ/ — The movement of /ʌ/ toward [ɔ]. /ʌ/ is the "short u" vowel, as in cut. People with the shift pronounce cut so that it sounds more like caught to people without the shift.