Runways are not closed for bulb changes. Closing a runway is a big hairy deal that is not something you do unless you have to.
Normally what happens if it is a multi-runway airport is that they wait until the runway is not being used because of cross wind conditions, then a guy in a yellow truck with flashing lights drives out and makes the replacement.
If there is only one runway, or if something has to be done to an active runway (like remove a turtle or something), then the FBO operator or tower just warns off any approaching aircraft on the CTAF while the dude in the yellow truck scoots out there and does it.
Normally they try to stay off the runway with the actual truck and minimize the time they are out there. So, for example, with a turtle what happens is the guy drives the truck on the grass until he is parallel with the turtle, then he leaves the truck's lights on, runs out, grabs the turtle, and runs off the runway again. He then drives the truck back (on the grass) and the FBO resumes operations.