Really?
Ex corporate IT guy here.
The first part is correct; the second part, well, that depends. You do NOT need to pay for new licenses if you stick with the current PCs. Are you running Windows Server Essentials?
The key question here is, "How much downtime can you afford?" Think about what would happen if the upgrade were to go wrong. What if it were to go horribly wrong? Think about the business aspects, not the technical ones.
This is, in fact, a lot of work. Each PC will have to be rebuilt, the RAM upgraded, and all the applications reinstalled and all the data restored (hopefully there's none). This is a time-consuming process. If you have all new PCs everything can be done off-site and the new PCs plugged in in one morning. And if something's not working, you just plug the old PC back in while you fix the problems.
Ask your IT support what changes she would make. For instance, if you have desktops or towers, could you switch to NUCs and save on desk or floor space? I presume you do rent your offices, and each square foot counts.
Another way of doing it is switching over gradually, perhaps 2 PCs at a time in your case. This is expensive in time, of course. A variation on that is to buy 2 new PCs and swap them in, then rotate the pulled PCs to the next 2 and so on.
Remember above all that this is a business issue more than a technical issue.