Converting my Free tier account to pay as you go account

Free tier is a standard AWS account only, it's just you won't pay for resources, which are free tier eligible and you get some free usage quota.

You will only need to set up the resources you need and will be able to use them. No additional steps.

https://aws.amazon.com/rekognition

There is no such thing as a "free tier account". Your account will be billed on-demand for any products or services that exceed the free tier offerings.

Please read: https://aws.amazon.com/free/

So in summary - just start to use the services you need to use - and you will continue to receive the benefits of any free tier use as per the documentation above.

Edit on your comment: You can just follow the process here Training an Amazon Rekognition Custom Labels model to train a custom labels model in recognition. It has nothing to do with free tier?

What do you mean that it is not enabled for your account - can you please provide any error messages that you are receiving when you try to perform this (or screenshots from the console).