Can I upgrade from S3Media Stream regular to Enterprise?

No, S3Media Stream Enterprise is a separate plugin not compatible with S3Media Stream (regular) since it is rewritten from scratch. Too many features in JW Player have changed since version 6.

Additional info: You cannot use both plugins alongside each other on the same site either because that will create a javascript conflict. But if you buy S3Media Stream Enterprise, you can keep using the regular version on your old site. There is a shortcode converter to convert old code to the new one if you want to migrate videos to the new site.  However, this needs to be done individually for each post or page.