In SGK you can upgrade build parts so they become another build part, this is done with wood build part that upgrade to stone versions.
To upgrade a build part the character must be holding the hammer item, then you look at a build part that is owned by the player and hold Right Mouse button and select the upgrade option. If the option is greyed out then that build part cannot be upgraded as it doesn’t have an upgrade build part set in the BuildPartList.
To set an upgrade build part go to the BuildPartList, then select the row of the build part you want to set an upgrade build part for like for example the StoneFoundation. Like in the image below.
Next in the Row Editor window you can click the Upgrade Build Part drop down and select a build part using the Row Name option. You can select any build part for the building to upgrade into but it is strongly recommend that the upgrade build part should be the same build type and size, if not you may cause problems with the system!
The cost of the upgrade is the Item Cost of the selected Upgrade Build Part to change this change the Item Cost of the selected Upgrade Build Part.
If you want an example of a working upgrade build part select the WoodFoundation row and look at its Upgrade Build Part.