Set User Rank via API

We recently updated our ranking system and use some custom ranking formulas for our new ranks. However, we have some longtime users who didn't meet all of our new criteria so they were "demoted" to a lower rank. We're hoping to grandfather these users into their old rank, but it seems like there's not any way to do that. There's no way I've found in the settings or rank settings and I've tried with a few users via the API with no luck. Does anyone know of a way to do this for a list of users that we want to manually change their rank even if they don't meet all the formula requirements? Or will the formula requirements keep us from changing their rank manually?

Here's the code I'm using to try to change the rank manually. I get a success message when I run this, but the rank doesn't change:

const options = {
  method: 'PUT',
  headers: {Accept: 'application/json', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    data: {
      type: 'user',
      rank: {id: '9'}
    }
  })
};

fetch('https://community.url.com/api/2.0/users/user-id', options)
  .then(response => response.json())
  .then(response => console.log(response))
  .catch(err => console.error(err));

 (I've changed our url and user-id for privacy, but I'm using the correct values in the code I'm running)