Notable changes from Old Spanish to Early Modern Spanish include: (1) a readjustment of the sibilants (including their devoicing and changes in their place of articulation, wherein voicing remains before voiced consonants, such as mismo, desde, and rasgo, but only allophonically), (2) the phonemic merger known as yeísmo, (3) the rise of new second-person pronouns, (4) the emergence of the "se lo" construction for the sequence of third-person indirect and direct object pronouns, and (5) new restrictions on the order of clitic pronouns.
Meanwhile, Judaeo-Spanish preserves some archaisms of Old Spanish that disappeared from the rest of the variants, such as the presence of voiced sibilants and the maintenance of the phonemes /ʃ/ and /ʒ/.
[3] From the late 16th century to the mid-17th century, the voiced sibilants /z/, /z̺/, /ʒ/ lost their voicing and merged with their respective voiceless counterparts: laminal /s/, apical /s̺/, and palatal /ʃ/, resulting in the phonemic inventory shown below: Spelling in Early Modern Spanish was anarchic, unlike the Spanish of today, which is governed and standardized by the Real Academia Española, a semi-governmental body.
There was no reference book or other authority writers or compositors could turn to, to find the "correct" spelling of a word.
That presents a challenge to modern editors of texts from the period, who are forced to choose what spelling(s) to use.