When you are deciding on replacement of your existing plumbing system, you should keep in mind the following factors in order to get the job done adequately.
Metal pipes are subject to corrosion over time, and this can be one of the reasons that you are seeking to replace them. If you are replacing the whole plumbing system, then your task is to choose a suitable pipe for the system, but if it is a partial replacement, then you should purchase pipe of the same material of the existing pipes. Having different types of pipes in the plumbing system, you might be inviting corrosion more rapidly. Along with the pipe, you will also have to look for other fittings like elbows, adhesives that will match with the existing system and will be able to tolerate the rigours of the system.
Apart from the material for the pipe, you will have to adapt to the local rules and regulations for residential piping. For example, PVC pipes are prohibited for hot water use at home; it is only meant for cold water. You can but use CPVC for hot water; but that too up to a specific temperature. Therefore when you choose the piping type, take into account all these factors beforehand.
When you are purchasing pipes for replacement be sure to buy a size a bit larger than the replacement portion, as there might remain a more significant gap between the pipes which will be difficult to join together; and if joined somehow will be at risk of leakage within a short time. Also when the pipes are cut for joining, makes sure that the end of the pipes are smooth so that the pipes are correctly aligned before they are joined together.
If you are replacing a cracked pipe, you should do it immediately, and before that, you should know where the cut off valve is located so that the water to the damaged area is completely cut off before replacement of the pipe takes place. Also, ensure that all electrical hazards are kept away from the water when such replacement work is on.
You could have to replace the main water pipe to your home; in that case, you have to ring 811 beforehand in order to prevent inordinate delay.
Since you have to hire a plumber for the job, it would be wise if you could take a few quotes from different plumbers. Make some discreet enquiry about the reputation of the plumber, or you can visit review sites to ensure the reputation of the company that you intend to hire is good.