I personally think selling a conversion kit for a new faction without existing products doesn't make a lot of sense from a business stand point. It's confusing product for new players and one of those annoying 'limited value' additional purchases for existing players.
I think the best option would be to include extra cardboard in the republic and CIS version of the expacks when they are released in anticipation of existing players converting old models. For example the republic ARC-170 expack could have 2 ships worth of cardboard for converting a rebel ARC model. That seems like the easiest option for a pseudo conversion.
That being said, I think even that is unfeasible as they would need to add that for no additional cost for the consumer. Which is a double edged blade as if they charge extra for the additional cardboard they most likely won't sell as well. If they do add it for free players will ask why they couldn't get that treatment for other expacks, shooting themselves in the foot.
Now speaking to the ships in question; the ARC and the Slave 1 are really the only 2 ships that could be released as is with a repaint. The sheathipede could probably fall into this slot too. The clone Z95 looks different as it was a specific model specially made for the Grand Army. The BTL-B Y-wings are also visually different and I think a lot of people would be disappointed (I certainly would be) if we didn't get re-sculpts of these ships to suit the new faction. I think if FFG have removed Nym from the rebels faction, its unlikely they will give him to the republic, and as the Scurrg was a prototype that Nym stole, it needs to be him in the ship. Asajj's Banshee is another ship that looks very different to its existing counterpart and is deserving of a re-sculpt.
Honestly I think FFG will try to make these 2 factions feel as new and unique as possible, and converting a bunch of old ships goes against that notion. We all know that the Y-wing and the ARC are givens for the Republic, but I wouldn't bet on the rest being converted into in the new factions.