Question: > 
> и**ме**на́ [ɪ**mʲɪ**ˈna]
> 
> 
> {vs}: и́**ме**ни [ˈi**mʲɪ**nʲɪ]
> 
> 
> 

At least according to IPA, these two "ме" sounds should be pronounced the same way. But I seem to notice that the "mʲɪ" sound placed before a stressed syllable sounds just like "mʲɪ", similar to "**m**ix" in English, whereas the "mʲɪ" sound placed after a stressed syllable sounds more like "mʲe", as in "**ме**ч [**mʲe**t͡ɕ]" -- albeit not as distinct.

Answer: 
Both of those syllables are in an unstressed position, hence the reduction of sound. It's very difficult or even impossible to spot any difference in this position. 

From the practical standpoint: **they sound the same**.
I would say that "mʲɪ" can be pronounced as the similar to "mix" (и) or as in "меч" (е), or even among them. Both options will be correct. It depends on the dialect of the speaker.