Hankal wrote:
Is an owner allowed to change (replace a defective) landing bulb.
Does it require an AP sign off?

Yes, an owner can do it, provided the owner holds a pilot certificate.
No, an A&P is not required. I do not see anything that permits an
individual non-pilot owner to perform preventative maintenance.

Chapter and verse is quoted below. Part 91 says who CANT. Part 43 goes
ahead and says who CAN perform maintenance.

Dave

§ 91.403 General.
(b) No person may perform maintenance, preventive maintenance, or
alterations on an aircraft other than as prescribed in this subpart and
other applicable regulations, including part 43 of this chapter.


§ 43.3 Persons authorized to perform maintenance, preventive
maintenance, rebuilding, and alterations.

g) The holder of a pilot certificate issued under Part 61 may perform
preventive maintenance on any aircraft owned or operated by that pilot
which is not used under Part 121, 129, or 135.


§ 43.7 Persons authorized to approve aircraft, airframes, aircraft
engines, propellers, appliances, or component parts for return to
service after maintenance, preventive maintenance, rebuilding, or
alteration.
(f) A person holding at least a private pilot certificate may approve an
aircraft for return to service after performing preventive maintenance
under the provisions of §43.3(g).


Appendix A to Part 43—Major Alterations, Major Repairs, and Preventive
Maintenance
(c) Preventive maintenance. Preventive maintenance is limited to the
following work, provided it does not involve complex assembly operations:

(17) Replacing bulbs, reflectors, and lenses of position and landing
lights.