It is compatible with Ck2 converted savegames, as well as with Vic2 to HoI4 converter, allowing the player to play a continuous megacampaign from CK2 to HoI4. It can be downloaded from this forum thread or from Steam. For instance I have seen references to 'The Kingdom of America' in localisation files if the USA is a monarchy (and some other name for a commie USA. The EU4 to Vic2 converter is a tool that converts a Europa Universalis 4 save and generates a Victoria 2 scenario (mod) from it. All of the localisation files concerning the displayed name of a country isn't in the same file. There is a modding project on paradoxplaza to make it work for all countries.I'm not so sure about that. I found entries for the USA, CSA, Britain and Germany and they're all named the same in all systems. The game has that capability, to change the country name according to political system, but it's not used. I still get references to dual monarchy in events, I don't really feel like searching through every event text and editing a word or two.
Some 10 files to edit, and I made custom flags, not much work. Technically you could just rename country in 2 or 3 files, but I created completely new country and just switched the decision for Austria to Create Danubian Federation instead of Austria-Hungary, to make sure I don't break something and that I can deactivate it quickly when I decide to play another country.