I have the same monster filter. Given that your pool guy is too lazy to do his job (though some pool services charge extra for this), you definitely need to open up that baby and have a look. Who knows when the previous owners replaced or even cleaned the cartridges? You've already slid by a year too long. When I bought my house with pool, having no knowledge of the previous owners' habits, my strategy was the same as yours: buy new cartridges (and o-ring) right away and start fresh with a known schedule. Cheap compared to the price of the house. It's your choice whether to examine the existing ones first, knowing you'll have to put them back in if they're in bad shape until you get new ones.
Having the same filter, I will pass along some pro tips. I'll presume you have the filter manual and know the obvious, like remove power from the pump and open the filter pressure relief valve to let air in and let the pressure equalize before loosening and removing the filter clamp.
Wear gloves. Those older tanks have nasty fiberglass hulls and they will irritate your skin. Disposable medical gloves are good for this. They will also keep your hands from turning black when you lube the o-rings. (Might as well lube the pump basket o-ring at the same time.)
Once you've removed the filter clamp and the top half of the shell, you can eyeball the innards, remove the top manifold, and pull out the cartridges. You have a big filter there, and those big cartridges are heavy when waterlogged, so be careful slinging them around. Stand them on end and inspect them well. They will be full of disgusting crud, but what you're looking for is tears or holes in the cartridges, or other obvious signs of wear like broken plastic bands or broken top/bottom parts.
After removing the carts, there will be dirty water at the bottom of the tank. Do not open the drain plug at the bottom front of the tank. Why not? (1) All that dirt will drain onto your equipment pad and then you'll have to clean it up. (2) Your filter is not young, and the threads for the plug have been known to deform, leading to leaks. (3) It's illegal in at least some parts of CA to release pool water where it can flow to a storm drain. Get the dirt out of the bottom some other way. I use a shop-vac.
There are various tools you can search for that speed clean these big carts faster than a garden hose alone. I like the Aqua Comb, other folks have different preferences.
Given the size of the belt clamp, it's pretty unwieldy and the only hard part of the process is getting it back on properly. For this part, it's easier to have an assistant to hold the opposite side of the clamp while you get it positioned and partially tightened. The assistant might as well hang around to do the tap-tap-tapping of the rubber mallet to help seat the clamp while you finish tightening.
After reassembling the filter and restarting the pump, you will probably see drips dropping from the clamp. Don't panic. This does not necessarily mean the filter is leaking. There will always be a few drips just from water being outside the tank. Dry the drips with a towel and give it some time to determine whether you have actual leaks.
Good luck, and happy filtering!
