Compression fittings are fully code compliant when installed in a non-concealed location. The copper supply piping in the wall is installed during the "stack out" phase of construction and at that time each fixture supply is stubbed out and capped. Later after all the walls are finished and the cabinets installed we do the final phase of the plumbing which is called the "trim out phase" which is when all the final fixtures are set in place. During Trim out the copper pipe stub outs are cut off and the line stops or angle stops are installed. Most plumbers, myself included, prefer to use the compression mounts for the line stops or angle stops because it eliminates any possibility of scorching a wall or cabinet which could result if we were to solder valves in place. The other advantage of compression mounted stop valves is that they can be very easily changed in future if the need should arise, although we constantly hear about people who run into problems here also. When a compression fitting is installed the compression nut is slipped onto the pipe first, then a brass compression ferrule is slipped on the pipe and the valve or fitting is then slid on the pipe until the pipe bottoms out in the fitting input port. With the fitting held firmly in place the compression nut and ferrule is then slid outwards towards the fitting and the nut is tightened in place on the fitting. Properly you should hold the valve or fitting steady with one wrench while tightening the nut with a second wrench. As the nut is tightened the compression ferrule is compressed into the wall of the copper pipe effecting a water tight seal. If it then becomes necessary to change the valve at a later time you hold the valve steady with a wrench while unscrewing the nut with a second wrench. Once the nut is off the valve the valve can be slid off the pipe. The nut and compression fitting will not slide off because the inside diameter of the compression ferrule is now slightly less than the outside diameter of the pipe on which it is installed. Rather than attempt to remove the nut and ferrule you remove the nut and ferrule from the new valve, slide the new valve on the pipe and tighten it in place with the existing nut and ferrule. In your case you have a small section of copper pipe in the basement that is leaking. You should not need to remove the valve or riser to the valve. Instead, you should be able to use a tubing cutter to cut out the bad section of copper in the basement and replace just that section by soldering in a new section of copper in place with a couple copper couplings.

1/4-turn ball valve replacements...........What to replace them with and how to replace them will depend on configuration of your existing supply-lines....the size and type of lines and existing connections. There are 1/4-turn ball valves available for literally any configuration. Plumbers have seen them all so it's easy for them to figure it out and replace them. One-timer do-it-yourselfers like me generally have to learn from scratch.....which is why I'm suggesting the plumber.....unless you're really, really interested in learning about it. It's one of those things that's easy and quick if you have experience and kind of a frustrating pain if you don't. For DIY, I would suggest shutting your water off, removing the existing valve and taking it complete to the hardware store to obtain equiv. 1/4-turn ball for your configuration. I suspect every one you have will be the same so upon figuring the first one out, you'll likely be all set to handle all the others as well. I would be surprised to learn things are otherwise at your house -- unless you have an older house that had replacements done already. In that case, you may have to examine them one-at-a-time. That's why I grump about the original contractors using the stupid el-cheapo valves in the first place. For an extra couple of bucks, they could use a decent fitting that would operate flawlessly and last for decades. As it is, many, many people like you and me have the annoyance and frustration of dealing with the inevitable failures of the inferior valves. In any event, if you're faced with a replacement....as you clearly are....my advice would be to replace with 1/4-turn ball-valve whether you choose to do it yourself or have someone else do it for you.

I live in a condo which has 122 units and ONE shut off valve for the building's water supply. In other words, if water to a unit has to be shut off for any reason, the whole building has water shut off. In my experience, you can't replace a shut off valve without turning off the water for the building. You use the shut off valve so that you don't have to turn off water for the whole building to change a faucet or even a toilet. I would double check with management or whoever is in charge of maintenance because based on personal experience, you would need to close down all the water to replace the shut off valves. Because of the horrible consequences if a shut off valve breaks, my building has an annual inspection of all the shut off valves in each unit. If your shut off valve is found to be leaking or in danger of seizing (e.g. not shutting or opening properly), our building will make note of it. The HOA then turns off the water for one day so all homeowners on the list can get them replaced and the HOA generally provides the ability to use a plumber who bids a discount price. But the chief incentive is that normally it costs $2000 to shut off the water but there is no cost if you have the valves replaces on that day.

(Assuming that you have no experience soldering copper pipes and have no desire to learn) (And assuming that we are even talking about copper pipes.) If you are going to tackle this project yourself, instead of hiring a plumber then you could use "Sharkbite" valves. They are more expensive than regular valves but you can just cut the pipe and push the valve on and you're done. They work with pex too, but with pex it might be more beneficial to buy regular valves and rent a crimp tool for a day to do it properly instead of buying a ton of sharkbites. Pex doesn't involve using fire like soldering copper does, so I feel like newbies would be more comfortable doing that.