Thanks for your feedbacks.
Currently, there is no upgrade possibility for purchased template licenses, as those are downloadable products, and not subscriptions.
Finally, as precision, the multi-site license is valid for all sites owned by the licensee. For customer-sites web integrators, one license per customer site should be purchased, which seems reasonable compared to the amount of work that has gone into our templates, and the amount of work that goes into them to keep them up-to-date. The licenses are lifetime, with lifetime upgrades...so they are really not expensive.
Maybe in the future we will have other purchasing/licensing models, but no change on templates licensing is planed for now.