I will warn everyone here against doing "free" jobs or completely off the books work. The reason is lawsuits.
You go and install a new light fixture for a neighbor and do it for no charge. 2 days later their house burns down and the fire marshall asks if they had any electrical work done recently and they say yes. The fire marshall determines the fire must be related to that, case closed. Your liability insurance will not pay because the job was off the books.
For the above reason alone, among others, always charge a nominal amount like $5 for changing the light and write an invoice. I know you're not going to do anything that you think will cause harm, but sh!t happens, so CYA.
