I seriously mean this. Here is my proposal for two countries to trade place: Mongolia and Serbia https://upload.wikimedia.org/wikiped...ia_Locator.png
Consider the populations and locations of Serbia and Mongolia, respectively. In total, they amount just about 10 million people. Location wise, both populations would find it very suitable in their exchanged places. For instance, Mongolia is between two rather expansionist powers, Russia and China, and could easily be wrecked by either if they decide to extend their interests to that country. However, Serbia has good relations with both countries, and no historical enmity with them. Especially with Russia does Serbia have a good brotherly relationship.
In addition, Serbia has huge issues with its neighbours, ranging from Albanians, to Montenegrins, to Croats and others. It has committed genocide and has a high amount of people who believe a Greater Serbia should be established in the regions of all of the aforementioned people (among others). Moreover, even they could be victims of the nationalist or perceived self-defence policies of the other peoples.
So, rather than expanding their territory through bloodshed and risking all they have, instead they can simply relocate to Mongolia (and rename it, of course), whereas Mongolians can locate to Serbia. Mongolians have zero enmity with any people in Europe, and have absolutely more than enough land and resources in Serbian territory. Their population is LESS than 3 million, but the size of their country is huge.
Both Serbs and Mongolians would have more peace in their territories. They would both be richer and safer, because they would not be at risk of invasions, and would have more trade (e.g., Mongolians would opt into the EU market easily because they'd be inclined to join the organization ASAP).
I sincerely hope this happens one day. Logistically, it is very easy. Imagine how wonderful it would be for them and the world!