I think the rational to change the packing is driven by the tower internal inspection criteria. Once every 10 years is what is recommended in API 510 Pressure Vessel inspection.
There is a discussion about vessel internals, “When vessels are equipped with removable internals, internals may need to be removed, to the extent necessary, to allow inspection of pressure boundary surfaces. The internals need not be removed completely as long as reasonable assurance exists that damage in regions rendered inaccessible by the internals is not occurring to an extent beyond that found in more accessible parts of the vessel.” that would allow the packing to stay in place longer as long as all the conditions are met.
I agree with the other comments concerning the fouling of the packing. Replace the packing and investigate why coke is being formed or entrained into the packing.