Is there an antonym for “capitalize” (as in letter-case)?

While I'll caveat that some people aren't fond of it being used as a verb, lowercase is frequently used in the manner you're referring to. For example:

Chicago style is to lowercase all of these. Chicago Manual of Style

or

Lowercase shortened, informal, or descriptive names of committees. KU University Style Guide

The verb is lowercase- defined by Merriam Webster as:

"to print or set in lowercase letters."

The standard options are "titlecase," "sentencecase," "uppercase," and "lowercase." Various abbreviations and marks for these are used in proofreading. It's somewhat specialized vocabulary because outside of publishing you rarely have cause to say things like, "this word needs to be lowercased."

If being proper / correct is not your greatest concern.. Say you simply wanted your listener to "Re-write this sentence withOUT Capital letters."… I'd wager a bet that asking them to decapitalize it - would give you as good a result - as any.

The alphabets may be referred as upper-case and lower-case. Another way to describe it is to say 'majuscule' (majuscular) for upper-case and 'minuscule' (minuscular) for lower-case.

Reference: http://www.synonyms.net/antonyms/majuscule