Yes you can later upgrade for minor fee. Just email us when you need to upgrade.
It is hard to write a solid definition like this and it would be better if you describe your use case and we can discuss it if you have doubts.
An example:
Lets say you deploy a CRM application and you want it to use a reporting server capabilities. You purchase enterprise scale license of jsreport and modify the CRM deployment to include a jsreport reporting service. You don't have to care how many servers you actually deploy now, because the enterprise scale license doesn't limit you in this.
If you later decide to enrich also ERP system with reporting service. You should buy another jsreport enterprise license, because this is a different application with different deployment.