Apologies to those of you who will receive two copies of this post.

What are you thoghts on this, folks?  My Romconlang sees various Latin pronouns being superceded / changing in meaning - generally in accordance with the developments in Gallo-Romance. For example:

'apud' replaced by 'ad'
'cum' replaced by 'apud'
'ob', 'per' , 'pro' and 'propter' replaced by 'por'
'ab' and 'ex' replaced by 'de'

and so on.

These prepositions of course govern various cases in Latin. Some of the cases (e.g Ablative) are not present in my conlang. In other instances the case still exists, but the 'meaning' of the preposition has shifted into a different case  - for example 'apud' governed the accusative in Latin, but is now not used in its original sense and has instead replaced 'cum', which governed the ablative.

So I have two questions in my mind:

Ultimately I have three distinct cases - a combined Nominative/Accusative, Genitive and Dative. What should happen to the ablative prepositions when, very early on,  I lose the ablative case? Morphologically speaking, the case merges wth the accusative (and thence subsequently with the nominative), so would any ablative-governing preposition stay as it was and become accusative by default, or would a speaker feel that that felt wrong, that it needed an indirect case, and shift it to the Dative?  Interestingly, the Latin division of the prepositions between ablative and accusative neatly mirrors the division in German between dative and accusative, which lead me to this question.

Secondly, when the meaning of a preposition shifts (e,g, apud), would there be a corresponding change in the case it governed?