Tag Switching.

[Alidrules]

Can you tag switch as all countries or are only certain countries allowed to do so?

[Marquoz]

Almost any nation can tag switch. For example, most countries can become Italy by changing their primary culture to an Italian one and then completing the province ownership requirements. BUT, if you're an end game tag, there are very few if any switches you can make. As an example, the Ottomans are an end game tag. If you start as the Ottomans, you can't form Italy, or the Mughals, or almost anyone else. The exceptions are end game tags that are elected HRE Emperor and are then allowed to unite the HRE into a different end game tag by completing reforms, end game tags that are Christian or pagan and form the Roman Empire, and Yuan (who is allowed to become the Mongol Empire).

[grotaclas]

In addition to what has been said before, the Papal States can't form the Roman Empire (I'm not sure if they can form the HRE if they can become a monarchy somehow). And there are German regional tags[eu4.paradoxwikis.com] which can't form each other (not even when forming other countries inbetween). Prussia is a special case, which can be formed by the others, but can't form them. And there are some countries which can't form certain other countries(e.g. England and Prussia can't form Rum). You can find this information in the wiki[eu4.paradoxwikis.com]

But you can disable end-game-tags in the options before starting a campaign. This will disable achievements, but then you can form other countries as an end-game-tag. There are still some restrictions though. The german regional tag restrictions still apply and for example the Papal states can form only few countries and the Ottoman empire can't form Byzantium directly(many more examples can be found in the game files and in the wiki(but the wiki often doesn't mention restrictions which only matter when end-game-tags are disabled)).

[Alidrules]

Cant assign both of you a win but thanks!