The advice you received about building an air-tight box cover for your recessed lights is still good.
I would caution you to verify that your existing fixtures are equipped with high temperature safety limits before deciding to do a simple lamp change over.
- LED lamps do not remove the fire hazard.
- They still produce heat so use recessed fixture covers that are large enough to keep any insulation away from the heat source.
Another major problem with older recessed light fixtures is air movement from the living space into the attic through the can light assembly.
Covering the cans
Can light covers can be made out of foil faced foam board, drywall, OSB or even aluminum flashing. Or you can purchase pre made covers like these.
Make sure the cover is large enough to prevent excess heat buildup.
- Most can lights have high temperature safety switches built in that will shut the lamp off if the temperature in the housing gets too high.
- Older fixtures may not have this feature and should be considered candidates for replacement.
- You should caulk or seal the cover to the floor of the attic to get a good air tight seal before insulating.
If you opt to replace the can lights you should consider installing ATIC-recessed fixtures. This stands for air tight, insulation contact to avoid the need for installing covers. You should be able to replace incandescent lamps with LED's without changing the fixture.
Insulating the rest of your attic
While you are in the attic sealing your can lights you might want to take some time and seal any other penetrations in the ceiling (including partition walls) to stop heat from escaping from the living space into the attic.
To stop cold air convection from penetrating your insulation material you should choose the densest material possible (cellulose over fiberglass in my opinion).
Regardless of the material selected make sure your contractor specifies insulation type, depth and density rather than just R-value or depth. If the density isn't right neither is the performance.
For more information:
Read "What is the safest and most effective insulation for our attic crawl space?" a Q&A answered by Ian MacLeod.