There is no therapy able to “rectify” the “degradation of insulating papers” criticality. Degradation is an irreversible process that can only be managed through appropriate prevention and/or mitigation actions.
Paper replacement is not a viable option because the cost would be close to the cost of fully replacing the transformer.
When the paper DP reaches a value of 200, which conventionally represents the end of thermal life, it is advisable to plan replacement of the transformer and modify the maintenance practices that may have contributed to ageing of the insulating papers.
The priority is to avoid degradation by intervening preventively on the symptomatic indicators.