Bohemia is a kingdom from the start. Furthermore, only electors or the emperor itself can be king rank inside the HRE. For example, if you are a member of the HRE and form Prussia/Germany/Italy (Netherlands?), this will bring you to king rank and make you leave the HRE if you are not the emperor or an elector yourself.
Because of this, Austria can't get Archduke count as king, because it would mean they have to leave the empire once they are not elected for one time. But what I don't know, does your nation get Emperor rank while you are the Emperor or is this just possible after Renovatio Imperii?
Another question would be what happens if you remove electorate status of Bohemia. If their rank is still King then, this would force them to leave the HRE, even if that maybe isn't intended.
And finally, if you form Italy while you are Emperor, but then loose an election, will this also mean that you then instantly leave the HRE?