I've been putting a j-box in the ceiling nearby and running new to the new light. As long as people don't mind a blank cover on the ceiling its the easiest way out and 100% compliant with the vendor's 90 degree requirement.
Does sleeving the existing conductor with a heat shrink tube make the conductor rated for 90 degree C?
Well, it's certainly not listed. This is only putting a band- aid on the problem anyway. The damage (and high temperature) often extends beyond the box into the cable itself and there is no way to fix that other than replacing the cable.
When I would take a light fixture down and find the insulation damaged from heat on the existing RH wires I would take my lineman pliers and strip some insulation off of the TW wire and simply slide it over the damaged conductor. and then for added insurance put some fiberglass insulation in the box between the fixture & the wires in the box. I would not recomend this for a fix on 90c fixtures. We used to do this when we would take down one of those drum type fixtures that enclosed the lamps in kitchens. They are only supposed to have 60 watt lamps in them but people overlamped them with larger lamps.