We performed upgrades of regular DTM properties to Adobe Launch via the "upgrade to Launch" feature. Now, we would like to use private extensions with those upgraded properties but they do not have availability of "development" by default. It seems that this cannot be changed once the properties are created (upgraded in our case). How can this be solved? Are there any alternatives?
I've had came to same situation earlier. Technically speacking there is no alternative when you migrate the properties from DTM to Launch. The "development" flag only available at the time of creation of new property.
But yes there is other alternative I've tried. You can create new development property for your private extension and then once you installed private extension in newly development property - the same extension will be available in other properties too respective to their extension catalogue.
This way you can ready to use your private extension in migrated properties.