Not all pension schemes are suitable for automatic enrolment. The pension scheme you do use needs to meet certain qualifying criteria, which is why it’s referred to as a ‘qualifying pension scheme’.
If you’ve an existing pension scheme and you want to continue using it for automatic enrolment, you’ll need to check that it meets the qualifying criteria.
You can check if your existing scheme meets the automatic enrolment qualifying criteria in The Pensions Regulator Detailed Guidance no. 4