Converting niche Instagram to personal account??

I did this very successfully. As long as your personal account is themed similarly to your niche it can be done very easily. E.g if you a skateboarding page and you want to convert it to your own because you do skateboarding yourself..an example like this will work I can explain more later.
But if you want to change a nature niche into your own but you do nothing outdoor/animal related then you have no hope.

You might lose some people but it's part of the game.

Many people build up an account with a picture of a hot girl then switch it.

It depends, because sometime for your theme account you connect with your followers and your followers know who you are, so you wouldn't loose as much

The way did it is that my account had say for example, 500 user animal photos. I then began uploading my animal photos every third one whilst deleting 20 user photos a day...so over a couple of months eventually it became 50/50 ratio. Then over the next month 80 % were my photos. Then I made an announcement that my account name has a copyright issue and ill be changing the name in the next week blah blah...gained plenty of sympathy in the post then apon the name changed people thanked me. I did this at 200K. Didn't lose any followers. The process was a bit more complex than this but in a nutshell this is what happened.

I've wondered the same thing