Many softener setups have a sediment pre-filter on the main line ahead of the softener. It is usually a cartridge inside a clear or opaque housing, and its job is to catch grit and sediment before it reaches the resin and the valve. The cartridge itself is a consumable, so it needs changing on a schedule. Whether you change it or hand it off depends less on the cartridge and more on the housing, the rating, and the shutoff valves around it.
Find out what you actually have
Before you decide anything, walk the line from the water meter or main shutoff to the softener. Note whether there is a filter housing between them, whether it has its own valves above and below, and whether a bypass is installed. Read the label on the cartridge or the housing for the micron rating. If there is no label, ask the installer to write the rating and the part number on a card and tape it near the housing. A card you can read without opening anything is the single most useful thing to have on hand.
The do-it-yourself line
You can reasonably handle the swap yourself if all of these are true: the housing opens by hand or with the wrench that came with it, the cartridge slides out without force, there are valves that let you isolate the filter from the water supply, and you can buy a cartridge of the same rating and length. If any of those is missing, you are not really doing a filter change anymore. You are doing a plumbing job, and that belongs with whoever installed the system.
When to hand it off
Call a technician if the housing is cracked or cloudy at the seam, the threads feel stripped or cross-threaded, the cartridge is stuck and you would need to pry it, there is no way to shut off the water feeding the filter, or you cannot tell which way the flow arrow points. Those are the moments when a quick swap turns into a flooded floor or a leak you then have to explain to the company that installed it.
The swap, in order
Set aside a towel and a bucket first. Close the valve that feeds the filter, or the bypass if your setup has one, so the housing is no longer under line pressure. Open a nearby cold faucet to relieve the pressure trapped in the housing, and wait until it runs down to a trickle. Unscrew the bowl slowly, and pull out the old cartridge. Look at the O-ring and the seat it sits in, and wipe away grit. If the maker calls for a particular lubricant on the O-ring, use only that lubricant. Seat the new cartridge the way the old one sat, check that the flow arrow matches the direction of flow, and tighten the bowl by hand until it is snug without forcing it. Open the feed valve slowly, then watch the housing and the fittings for seepage. Run a cold faucet for a short time to clear air, and check that the water is running steadily before you walk away.
Ask before you commit
Whoever sells or installs the softener should answer these questions in writing. Which micron rating did they install, and what replacement cartridge should you buy? Where will you get the replacement, and do they keep spares? Will they show you the swap once while they are there? Does the warranty paper say anything about opening the housing yourself, and if it does, what does it say? These answers settle most of the decision before the first cartridge ever comes out.
Keep a simple record
Write the date of each swap on a strip of tape on the housing, and note the cartridge rating beside it. When the next swap comes due, you will know whether the last one was done on time, and any technician who visits can see the history at a glance. If you want a broader record of the rest of the system, the same habit works well alongside the notes you keep for the softener itself. For a wider look at how reviewers weigh these service questions, see How We Rate Water Softener Companies.