@barnee
When an Industrial Complex has been converted into an Underground Factory, there is no reverse.
Exception:
When an Underground Factory is moved, the Underground Factory Upgrade is lost - and so is any upgrade to Heavy Industry.
When the Industrial Complex has been moved to a new location, any upgrade to Underground Factory and Heavy Industry can be purchased again.
If the enemy captures an Underground Factory, this Underground Factory is captured as such and the Underground Factory capacity is still intact.
Remember:
If an Underground Factory has been upgraded to Heavy Industry, this upgrade is lost if captured by enemy forces.
Only the original owner of an Original Industrial Complex can upgrade it to Heavy Industry.
This rule also applies to Underground Factories.