Well... it really isn't difficult to add those designations. I mean, fundamentally, it's a matter of changing a few lines of text. If anything about it can be called challenging, it's that I'm not sure what would be preferable for all the other designations. That is to say - the three ships from WC4, the Banshee, Avenger, and Vindicator (which, oddly enough, doesn't even appear in UE) all have designations from Arena. But the Border Worlds in UE have a whole bunch of other fighters, too. The Scimitar, Epee, Ferret, Bearcat, and so on. What I'm really trying to make up my mind on - and would very much welcome other people's opinions - is whether each of these ships should get a new designation similar to the ones for the WC4 BW ships, or should they instead revert to their original Confed designation?I think if you were to ask someone like me, they'd say it'd be fantastic to add the Arena material in. Now from a slightly more practical perspective, it's not that big a deal. You certainly wouldn't want something like that to hold up the whole project, but it'd be nice icing on the top if it was easy enough to do.
Fifteen years ago, I clearly was of the mind that the Border Worlds would at some point have established a total designation system encompassing all their equipment, no matter how old. But all the historical entities that inspired me in imagining how the BW forces would develop over time, actually did things much more chaotically, on a more ad hoc basis. Some countries liked to give their own designations to most planes. The British liked doing that. But even the British eventually gave up on it, leading to a bizarre situation where early model Avengers served the British as the Grumman Tarpon, whereas late models were Avengers again. Other countries would sometimes make up local designations if they were going to produce the aircraft locally on license, while sticking with the foreign designation if they were only buying aircraft from the overseas manufacturer.
A bit of chaos like this is actually nice, in theory... but might not work out too well in practice. What works in reality often just makes things confusing in fiction, where most people like to see more consistency than can be found in reality. So, at the moment, I'm trying to decide what's best for all the other ships, if I do use the Arena designations. I'm inclined at the moment towards the solution where, at least old ex-Confed aircraft that were no longer in production when the Border Worlds formed (the Scimitar most definitely, the Ferret and Epee maybe) should retain their original Confed designations, because there would have been no point in giving them new designations. Others, like the Bearcat, might have been given a new designation, or they might not, because again, no production was taking place - after all, we have no reason to believe the Border Worlds would ever get more Bearcats than they stole in 2673, and even them retaining the ones they stole is a somewhat controversial idea. Only the brand new fighters that were somewhere just over the horizon, such as the Rapier IV, should definitely get a new Arena-style designation.
Also, with the Scimitar in particular, it's hard to even say what the Confed designation should be . The only official designation we have for it is from the Confed handbook (CF-105, IIRC). Did the Scimitar get a new Confed designation at some point, as it seems the Broadsword did (provided the Handbook Broadsword is the same as the WC2 Broadsword)?