Term for when a negative word is used positively?

You could be talking about semantic change also known as semantic shift, semantic progression or semantic drift.

the evolution of word usage — usually to the point that the modern meaning is radically different from the original usage. In diachronic (or historical) linguistics, semantic change is a change in one of the meanings of a word. Every word has a variety of senses and connotations, which can be added, removed, or altered over time, often to the extent that cognates across space and time have very different meanings. The study of semantic change can be seen as part of etymology, onomasiology, semasiology, and semantics.

The linked Wikipedia article lists example words such as awful, demagogue, knight, etc.

Elevation: e.g., knight "boy" → "nobleman".

Auto-antonymy: Change of a word's sense and concept to the complementary opposite, e.g., bad in the slang sense of "good".

Auto-antonym is a sub-category of semantic change:

An auto-antonym (sometimes spelled autantonym), or contronym (also spelled contranym), is a word with a homograph (another word of the same spelling) which is also an antonym (a word with the opposite meaning). An auto-antonym is alternatively called an antagonym, Janus word (after the Roman god), enantiodrome, self-antonym, antilogy, or addad (Arabic, singular didd). It is a word with multiple meanings, one of which is defined as the reverse of one of its other meanings. This phenomenon is called enantiosemy, enantionymy or antilogy.

[Wikipedia]

Related to Rathony's answer—semantic change, the linguistic term amelioration specifically refers to words whose meaning is elevated, improved, or made positive.

Examples of such words, used especially in slang, are:

• geek (see ‘Word Story’ notes)
• nerd
• wicked
• (the) dog's bollocks
• sick
• bad, badass
• dope

In linguistics, amelioration is the upgrading or elevation of a word's meaning, as when a word with a negative sense develops a positive one. Also called melioration or elevation.

Amelioration is less common than the opposite historical process, called pejoration.

Amelioration, whereby a word takes on favorable connotations and deterioration whereby it takes on pejorative associations, are often telling indications of social change. […] Hence, villein, a medieval serf, and Anglo-Saxon ceorl, still lower in the hierarchy, deteriorated to villain and churlish, while noble and gentle, predictably, rose in moral connotations.

_{Geoffrey Hughes, Words in Time: A Social History of the English Vocabulary}

Source: Grammar About.com

An extract taken from Daily Koss

Origins of English: Amelioration and Pejoration

Amelioaration ... A more recent change can be seen in “geek” which in 1916** was listed as U.S. carnival and circus slang meaning “sideshow freak.” The word appears to be a variant of “geck” which in the 1510s meant “a fool, dupe, simpleton.” By 1983, “geek” was used as a slang term referring to students who lacked social graces but were obsessed with computers and new technology. In the twenty-first century, “geek” has expanded to refer to someone with special knowledge, not limited to computers.