there are stories about softening old rubber with silicone, washing powder or sunscreen … I don’t believe them. Once the molecular structure of the rubber is destroyed you won’t get it rebuilt by any additive; you may fill in cracks or make it feel slippery, but it won’t help.
As long as your screens are out anyhow and the gaskets are still available at a reasonable rate I’d replace them. If you don’t want to, but dislike wet feet and rusty floor pans, you might use the old rubber, but reseal it with some kind of sealant that was used on a car of mine some 15 years ago and still works fine.
It was a 3M product, it came in a 400 ml can to be mounted in the type of silicone pumps, it was black and supposed not to harden, but to remain elastic. It was inserted over the nozzle between the glass and the rubber and betwen the rubber and the metal lip.
75 XJ6L 4.2 auto (UK spec)