Once you have passed Erbkaisertum, you can do whatever you like with your nation. You won't lose Emperorship (I'm pretty sure that if you do it's a bug). The one thing you cannot do is switch the country you're playing, i.e. releasing and playing as a vassal or colonial nation. Tagswitching (i.e. forming Prussia), government form switching, even religion switching, it's all open to you once you are sitting safely on Erbkaisertum (potential losses to Imperial Authority and prince willingness to accept reforms may still apply).
Important note: Erbkaisertum passed is not exactly the same state as having a hereditary HRE, the latter happens when there are no electors left iirc. What happens in the second case I'm not sure.