In general, one can replace fixtures freely, subject to a few
	caveats.  First, of course, one should check the amperage
	rating of the circuit.  If your heart is set on installing half
	a dozen 500 watt floodlights, you may need to run a new wire
	back to the panel box.  But there are some more subtle
	constraints as well.  For example, older house wiring doesn't
	have high-temperature insulation.  The excess heat generated by
	a ceiling-mounted lamp can and will cause the insulation to
	deteriorate and crack, with obvious bad results.  Some newer
	fixtures are specifically marked for high temperature wire
	only.  (You may find, in fact, that your ceiling wiring already
	has this problem, in which case replacing any devices is a real
	adventure.)

	Other concerns include providing a suitable ground for some
	fluorescent fixtures, and making sure that the ceiling box and
	its mounting are strong enough to support the weight of a heavy
	chandelier or ceiling fan.  You may need to install a new box
	specifically listed for this purpose.  A 2x4 across the ceiling
	joists makes a good support.  Metal brackets are also available
	that can be fished into ceilings thru the junction box hole and
	mounted between the joists.

	There are special rules for recessed light fixtures such as
	"pot" lamps or heat lamps.  When these are installed in
	insulated ceilings, they can present a very substantial fire
	hazard.  The CEC provides for the installation of pot lamps in
	insulated ceilings, provided that the fixture is boxed in a
	"coffin" (usually 8'x16"x12" - made by making a pair of joists
	12" high, and covering with plywood) that doesn't have any
	insulation.  (Yes, that's 8 *feet* long)

	NEC rules are somewhat less stringent.  They require at least
	3" clearance between the fixture and any sort of thermal
	insulation.  The rules also say that one should not obstruct
	free air movement, which means that a CEC-style ``coffin''
	might be worthwhile.  Presumably, that's up to the local
	inspector.  [The CEC doesn't actually mandate the coffin
	per-se, this seems to be an inspector requirement to make
	absolutely certain that the fixture can't get accidentally
	buried in insulation.  Ie: if you have insulation blown in
	later.]

	There are now fixtures that contain integral thermal cutouts
	and fairly large cases that can be buried directly in
	insulation.  They are usually limited to 75 watt bulbs, and are
	unfortunately, somewhat more expensive than the older types.
	Before you use them, you should ensure that they have explicit
	UL or CSA approval for such uses.  Follow the installation
	instructions carefully; the prescribed location for the sensor
	can vary.

	There does not yet appear to be a heat lamp fixture that is
	approved for use in insulation.  The "coffin" appears the only
	legal approach.