It seems like a fairly simple job so I am debating doing it myself. However, giving the age, is there anything I should be concerned about?
Would the threads be aged to make removal difficult? What about inserting the new plugs? The biggest risk seems to break a plug during removal or insertion.
The real question is: should I attempt this myself or take it a shop?
I'm sure the cylinder head on this vehicle is Aluminum and the spark plugs themselves are steel. So you do have the potential for corrosion to seize these into place making replacement difficult.
I would definitely get a good quality spark plug wrench so that you have the best chance of getting these out. Also some penetrating oil (I like PB Blaster) applied and allowed to penetrate overnight often helps. But don't go overboard on that. Too much will drain down into the cylinder once you get the plug loose and too much can be a problem.
Worst case is that one or more of them either come apart or pull the threads out with them. It happens and there is little you can do other than get the plug threads fixed or get a replacement head.
While I agree with @jwh20 on what could go wrong, this is a routine maintenance item which is commonly doable by (my subjective opinion) almost anyone who can change oil.
If you are careful removing the plug (e.g. if it is stuck - just above the job and then have someone else do it as you will be able to drive it to the shop), I would definitely consider this to be manageable risk level.
On almost all cars spark plugs are easy to replace with some basic tools and a plug socket, and almost anyone can do it. You'll want to make sure the area around the plugs is clean so you aren't dropping grit into the engine. As others say a squirt of penetrating oil a few hours before is a great idea. It is normal that little bit of force is required to get them to free, they aren't finger tight.
I'd be shocked if your 22 year old car was still on its original plugs, depending on mileage and how regularly it's been maintained it may several sets in. Unless you suspect this car has been neglected there's no reason to think that your plugs will be seized. They may not even need replacement at all, but there's no harm in checking as you can just put them straight back in if they are good.
Other things to check while you are looking at the plugs are the spark plug cables and the plug sockets. Any signs of wear, fraying or oil soaked cables mean they need replacement. The sockets that connect to the plugs can get gunk or corrosion on them, if it's there it should be visible and you can clean it off with some electrical contact cleaner and cotton swabs.
