Restructuring your business is a huge responsibility. Not only do you have the livelihoods of your employees in your hands, you also have the future of your business. They key is to take an objective look at your aims and goals over the long-term, and not be caught up in office politics, favouritism, or sympathy for those who work for you. It sounds harsh but it's the right way to get your business back on track.
Firstly, you need to think about the different roles within the company. Which roles are necessary and which are superfluous? Sometimes jobs get created and after a while they're simply not needed any more. Sometimes an external circumstance means your business starts to lack a particular aspect - if all your customers have been asking for a certain service, why don't you offer it yet? The restructuring allows you to get rid of roles that don't perform well, and to create roles that would drive the business in a great new direction.
The second thing to do is to outline a job description for each of your roles. Include the duties that the employee would have to carry out, who they would liase with as part of their role - for example, will they work closely with people in another department or will they manage staff, or will they report to management? These job descriptions will help you see how your company will operate and give you a chance to see if there are any roles you've missed out. For example, is one particular job description too long and complex? Perhaps this would be better as two roles, or perhaps you should create a secretary's position so that several staff have some help with their admin.
Now you're in a position to take a look at your existing staff. Which of them have both the skills and relevant seniority to fulfill the new roles? Create a pool of potential candidates for each role. If some roles are direct swaps that's great but it could be the case that some people will have to take on different responsibilities. Talk to managers and find out which people would be best suited to the job.