I had reconstructive surgery with silocone breast implants 20 years ago. Should they be replaced?

Is something wrong? If there is nothing wrong with your implants, they do not need to be replaced. However, silicone gel breast implants can wearout, and if the leak is contained within the pocket that normally forms around the implant it can be difficult to tell if there is a leak. This is called a silent leak. The best noninvasive way to check them is with an mri.
Talk to your surgeon. The current fda recommendation is to change silicone implants every 10 years. See your surgeon so he can review your implant type and talk to you about the possibility of replacing them.
Probably yes. You had your implants with the older generation of implants which had a more fluid gel in them. When they rupture, it is difficult to remove them because the gel sticks everywhere. It is preferable to replace them before they rupture, and get new generation of silicone implants with more cohesive gel. Surgery would be much easier. Remember implants are not lifetime devices.
Depends. Breast implants are not meant to be life-long devices, which is why most companies guarantee them for only 10 years. If you have not had any problems (capsular contracture, leak, etc.) with your current implants and they have not been placed on recall (like the french pip), I do not see a reason to remove them. You should continue to do self breast exams and get mammography to screen for cancer.