1999 Buick Century maintenace

I don't know what problems it could cause. Seems to me clean transmission fluid and a new filter is always a good idea, especially for an elderly transmission. I've heard it said, and believe it's good insurance, anyway, that you should change your fluid and filter whenever it's no longer cherry red, which for me is about every other year, manufacturers' suggested intervals of several times that notwithstanding. I would, however, make sure I used Dextron that says it's especially for older transmissions. Castrol has one. It costs more, especially now, but I figure anything I can do that might forestall a transmission overhaul is worth it. The common grade stuff seems too thin somehow.