Does 'evolution' means 'upgrade' ? 'downgrade' or just 'gradient'?
I wouldn't associate evolution with upgrade or downgrade specifically, since it means:
In other word, it changes its state (up or down). Evolution is not linked to a particular trend. As such, evolution is more closely related to "gradient", even though the former is about the process, the latter about the value.
Note: you can find a loose association between those terms (evolution vs. downgrade or upgrade) in the software field, where a soft evolution usually means upgrading to the next version.
John Y says in the comments:
True. But "positive" doesn't mean just "beneficial" or "more advanced".
If the evolution of graphic cards noise level is one of a growth (a positive increase of decibel levels), I wouldn't equate "evolution" to an "upgrade", but rather a "downgrade" in "noise pollution".
Usually, evolution implies a gradual or modest upgrade, though it can also refer to any change, especially those that occur through natural processes, or are inevitable through the passage of time.
In the case of "unnatural" evolution (such as the new release of a human-made product), evolution is intended as a definite upgrade, though not a huge one. (If it were huge, it would more likely be called a revolution.)
I would say evolution means adaption, since it is unbiassed. The concept I have learned is that whenever the environment changes the organisms need to adapt using the mechanisms of mutation and selection (survival of the fittest). This could mean an upgrade as well as a downgrade of 'features'. Please consider that 'features' like creativity or intelligence do not prevent us from destroying the planet, thus evolution may lead into a dead-end.
Although in a non biological context evolution is also used as a synonym for the next generation (technical innovation) which would mean an upgrade but never a downgrade.