There is a chance the post will go down on search results. But if changing it, make sure the new title contains your main keyword. If the post is doing super well (top 3, let's say), leave it as it is. If it's not broken, don't fix it.

Typically any change to a title means that google needs to digest the change. That often results in a temporary change in your SERP position.

If you are changing titles to use better key words that bring more traffic, perhaps it's a good idea. Otherwise, I would agree, if it's not broke, don' fix it.

Ya for a time being there are chances the position might have a fallback. But, if it's worth it (includes focus keyword) then do make the changes in your title but don't forget to get your post indexed again.

By changing the topic of a particular blog it won't affect the ranking of the post. Just the thing to keep in mind is that it should be relevant enough and should be eye-catchy. Make sure to maintain the need of the reader in your blog post that you promised in your title else your bounce rate will start to increase.