This is an outstanding question, and one with some level of complexity. There are several strategies, but to outline the change over it is important to know if the workers were correctly or erroneously characterized as independent contractors prior to the change over. As you probably suspect, the change might trigger recognition of prior mischaracterization, or at least claims of such. It is therefore important to manage the potential liability created by the change over.
This is not the kind of thing that can be addressed with any level of reliability on a public internet site. You need confidential advice from an attorney who knows much more about the underlying situation. You really need to locate and consult with an employment law attorney who can work with you to best make this transition without creating substantial exposure for you.
Good luck to you.
You will need to get the appropriate documentation from them (social security numbers, etc.) and place them on payroll. You must deduct appropriate taxes, pay for workers' compensation insurance and follow labor laws. Consult with an employment law attorney. You can find many qualified ones who work for reasonable fees on .com.
Transferring independent contractors to regular employees is a very delicate situation. If their responsibilities, what equipment they use, and other I.C./employee factors have not changed, you will want to be very careful not to give these persons an opportunity to sue you for incorrectly labeling them independent contractors in the first place.
You need to speak with an experienced employment wage and hour attorney to discuss whether this is advisable and how to go about it.
It was a very good idea for you to seek out guidance.