Anyone that recommends changing a shaft seal before it fails is trolling for revenue from the uninformed. If it isn't leaking, leave it alone. Are you certain they are actually recommending changing the seal as a preventative maintainence item?
The service chart in the manual clearly indicates the seal should be replaced at 2 year intervals. I'll just disregard it and wait for actual indications of wear. Thanks.
If you were using the deck in commercial service 6-8 hours a day, 300 days a year, it might make sense to replace it after 2 years while doing other maintenance. It says the same thing in my manual. Not leaking after 7 mowing seasons.