I don't know Persian, however I have some knowledge of linguistics. The example given seems to be linked to a shift in register (different use of language in different circumstances). The formal form is 'greater effort'; the informal form is 'lower effort' / more relaxed, which here is a shift from the higher pitched 'a' sound to the lower pitched 'u' sound.
In the example given, this seems to be a semi-generalised rule for this particular sound. I'm not sure if English has similar semi-generalised rules, but something slightly similar would be the general use of contractions. To some people, contractions are a marker of informality and should not be used in highly formal speech or writing.
- 'will not' vs 'won't'
- 'can not' vs 'can't'
- 'should not' vs 'shouldn't' (I avoided using 'shouldn't' in the above paragraph)
In general, contractions are dropping or altering specific sounds, which seems to fit with the original poster's request. As well as contractions, some other examples of shifts in register are:
- 'want to' vs 'wanna'
- 'going to' vs 'gonna'
- 'yes' vs 'yeah'
- 'no' vs 'nah'
You will note that the informal form is often 'less effort' than the formal form.
However, be careful not to label it 'lazy' or 'incorrect'. Often the formal form is regarded as 'formal' because it is the type of language (dialect) used by the power group in that culture. Formal language is often used as a tool to exclude others from membership in that power group, or to mark out membership / signify access to education / parentage / etc.
Sometimes the use of a differing dialect is deliberate to signify that the speaker refuses to participate in what they see as an unjustified power structure. (Or to signify membership in a different social grouping.)