Where is the "Upgrade to Map Stash Tab" option?

Already bought a handful of tabs.
When I bought them, "Map Stash Tab" didn't exist even though it was sorely needed.
Now it's here and I want to transform one of the tabs I bought previously to store maps to a dedicated map stash tab. There's no such option.
Why isn't there such an option?
There HAS TO be the option to transform a regular stash tab to a map stash tab!