I have seen plenty of people asking how to do this. The process isn't very neat and is easy to mess up so it is advisable to save before doing it.I will assume that you can use console commands. There are plenty of tutorials for that. Here it goes.
Let's take Germany and France as our example. You're Germany and want to switch to France. Use console command addplayer to control France. As France offer Germany any province, say Calais. As Germany accept the offer and then release yourself as any nation in Calais(make sure that you have ticked the play as released vassal button and that you are only giving that one province. I have occasionally messed this up). Then as Calais offer a union to France(not the other way around if you wanna have the French tag, weird I know). Accept the union and voila you are in single player again. If you wish to change your flag back to normal the n just switch your ideology and then switch it back.(if you are a poor nation you can use the formable nations editor to do it in a relatively cheap way, though it is a bit more tedious. Go to Editor>Map editor>Whichever map you use>Formable nations. You can probably figure out the rest. If you can't just ask.)
If this was too confusing then you can ask me what you don't understand. I can probably make a video tutorial if you want.(Due to timezones and the fact that I don't check everyday, you might have to wait a while for the reply).
You can use this method to avoid border gore, make your vassals join your alliance to allow your allies and other vassals to pass through them, change their ideology(though there is a simpler way to do that), make them form a certain nation when your allies just refuse to form a nation for whatever reason, add a little flavour to your game or simply to switch to a nation which you find interesting in any of your game (and probably a lot more).
By Beter Griffin
I have begun work on a probably 1000+ province map of the Roman world. Unlike the hoi4 map project, this one will not be abandoned. In fact, I might even restart the hoi4 map someday.
Map I'm using:
Growth rate, regions, etc.: 10%
Fleshed-out scenarios (events and other things that will require more than /map): 0%
the file was too big to attach and I had posted on reddit first.
I want to inform you about the development of a new scenario. It will be like " another world."
What will happen in this scenario? First, the action will take place before WW1. The United States and other countries will support the independence of not only Ethiopia and Liberia, but also other colonies. Africa will be free, Asia-partly South America, North America. Portugal would retain some of its colonies, but Angola and Mozambique would become independent States. Great Britain will lose a number of dominions, with the exception of the British Raj. And the colonies will get the status of dominions. France would lose its colonies and also give dominion status to countries: French North Africa, French Morocco, French Guiana. With Spain, the situation is the same. The Qing Empire will not lose cities: Hong Kong, Macao, Qingdao, etc. because of this, there will be no civil war.event "the fate of Hong Kong and Macao", a new vassal "Portuguese Raj" and an independent country "French Raj"
Additionally* there Will be a new event "African revolt" (for France). There will be a civil war in Spain. There will also be a new event, "the beginning of WW1". Because of the Austro-Serbian relations, WW1 will start again
Due to the fact that the scinario done in Addon+, there are some problems that I will solve in the near future.
I hope you enjoy this scenario. Sorry about my English.