The change in spelling from i to j was not connected to any immediate change in pronunciation. Words like iamais, iambon, iour were already pronounced with /ʒ/ at that point, according to the Wikipedia article "Phonological history of French" (not really a reliable source, but it agrees with what I remember reading elsewhere about this point).

The Old French ancestors of such words most likely contained an affricate /dʒ/, like the j in English jump, rather than the fricative /ʒ/, although I think there is a bit of uncertainty about this.

I don’t think that /j/ was used at any time during the period in which we call the language "French". Ultimately, some cases of "j" do go back to /j/ in Latin, such as in jus from Latin ius /juːs/, although others go back to g as in jambe from Latin gamba or z as in jaloux from Latin zelosus.

The sound that is now /ʒ/ was never pronounced like /h/ in French. The pronunciation of Spanish j as a voiceless fricative /x/, /χ/ or /h/ is a further development from a series of sound changes from /ʒ/ to /ʃ/ to /x/.