By Googling "contraction antonym oxford" Oxford offered the following:

contraction: shorten (a word or phrase) by combination or elision. "these sources were called quasistellar objects, which was soon contracted to quasar"
synonyms: shorten, abbreviate, cut, reduce, abridge, truncate "the name ‘Jacquenard’ was soon contracted to ‘Jack’ in English"
antonyms: expand, lengthen

Search for "expand contraction" let to a number of grammar sites giving instructions for dictation; there was also a lead to

Thomas Middleton and Early Modern Textual Culture:
By John Lavagnino p762 (p731 also) expand, expanded, expansion,

'later Crane texts expand contractions more than early Crane texts.'

I could find no word to describe the interpretation/ filling out of sigla which is a similar process.

By Googling "contraction antonym oxford" Oxford offered the following:

contraction: shorten (a word or phrase) by combination or elision. "these sources were called quasistellar objects, which was soon contracted to quasar"
synonyms: shorten, abbreviate, cut, reduce, abridge, truncate "the name ‘Jacquenard’ was soon contracted to ‘Jack’ in English"
antonyms: expand, lengthen

Search for "expand contraction" let to a number of grammar sites giving instructions for dictation; there was also a lead to

Thomas Middleton and Early Modern Textual Culture:
By John Lavagnino p762 (p731 also) expand, expanded, expansion,

'later Crane texts expand contractions more than early Crane texts.'

I could find no word to describe the interpretation/ filling out of sigla which is a similar process, in which the whole page may be transliterated with sigla or in full.