You are effectively dismissing them from your employment, so you will have to have good reason, or those who have been on the payroll for a year or more will be able to sue you for unfair dismissal.
Of the six acceptable reasons for dismissal (conduct, capability or qualifications, illegality, redundancy, reaching the default retirement age, or 'some other substantial reason'), most are inappropriate if you wish to continue to use these people on a self-employed basis. You will probably have to rely on 'some other substantial reason'.
You will also have to persuade HM Revenue and Customs that your employees are genuinely self-employed, otherwise you will still be liable for their income tax and National Insurance (NI) contributions. One of the factors the HMRC is likely to take into account is the extent to which their income comes from a variety of sources, so you will not be able to rely on using them on a full-time basis.
One way and another, this is a more difficult move than it might at first appear. Think long and hard, and take good legal advice before you make it.
