If you are contract employees, then your contract itself would control how you are to be terminated. If there is no provision in that agreement stating that a particular time frame of notice would have to be given, then none would be required.
Now, assuming that they somehow converted you to their employees when they changed your pay structure, that would actually not change whether or not some notice or reason has to be given. In employment law, if you do not have a contract specifically stating that you can only be terminated for cause, or that you can only be terminated following a specific amount of notice, then your employment is "at will." At will employees can legally be terminated at any time, be they employees or contractors.
There would be no legal requirement to release everyone. Terminating just one would be legally fine, as long as there is no evidence of a discriminatory purpose behind the decision to terminate...and by that, I mean that you'd have to specifically prove that the actual reason for the termination was race, religion, gender, age or disability (those forms of discrimination made illegal by statute).
As for the lack of pay, Virginia is not a great state for that to occur in, because your state's Department of Labor doesn't process wage payment claims. Instead, they force you to deal with it yourself in small claims court, so that's what you'd have to do, in terms of getting your owed wages. Send the employer a demand letter, requesting payment by a specific date and, if they don't pay by then, file a complaint in small claims court.
Now, if there were other overtime issues (they never paid it, because you were contractors) then it is possible that the change in your pay structure could have changed your status to the point that the employer owes you back pay for unpaid overtime. You can file with the state's Department of Labor for that issue.