How to keep a product maintainable?

How do you know if you have the ability to maintain your product?

With complex systems, having all the design files and source code does not always mean a lot.

Do you know how to build the source code?

Do you know why the hardware is designed the way it is?

Do you know how to test and verify changes?

Do you regularly use these tools?

The only way to know for sure if you can efficiently make a change to your product is to practice doing so.

If you don't, things soon ossify to the point where the cost and risk of any change are prohibitive.