Should I let my dentist replace my amalgam fillings? My dentist seems to be pretty insistent on replacing my amalgam fillings. I have had no trouble with them up to this point. He says that the fillings are old (some 25+ yrs) and will shrink and leak.

Depends. It depends how the fillings appear in the mouth and if they are defective. Also, the way how your bite comes together is important. If you are not sure, get a second opinion. Changing out good amalgam fillings are not recommended.
Replace if bad. If he can show you recurrent decay or broken fillings, then yes you should fix them, but you should also have the option of knowing that there are optional filling materials that you should be able to choose: silver, gold, plastic or porcelain.
Old is not a reason. Amalgams do just fine for decades upon decades. They also corrode around the periphery and in a sense seal the edges. It must be demonstrated that it is failing. I really dislike the notion of dentists pushing for tooth colored fillings because amalgams contain mercury (it is stable once mixed), removing them poses a worse threat by volatilizing it. Unethical to remove if old but serviceable.
Not without reason. Unless they are demonstrating decay , the choice is yours. If they are leaking, you will get decay and breakdown and should be treated. Let's see the evidence before replacing. I have amalgam fillings from 1964. Unless i start seeing decay, or they start to break, they will stay. Maybe you need to get another opinion. See a prosthodontist , preferably 40 or older. More experience with old amalgams.
If they need to be. It is possible that a 25+ yo filling needs replacement.. Let's face it, nothing lasts forever. But all of them at the same time? If you are that concerned, get a second opinion.
Often . Often there is a justification to replace amalgam fillings. The decision should be based on the integrity of your fillings, and your preferences regarding cosmetics. If your amalgams are wearing, chipping away, cracking, corroding, or if there is tooth decay around them - replace them. If the cosmetically pleasing tooth colored fillings would make you happier - replace them. On the other hand, if you are content with the appearance of your teeth - and the fillings are stable and intact - then don't. Note that the average life span of an amalgam filling is about 12 years - so you have gotten double your mileage out of them! often a dentist can use intra-oral photography as an educational tool to show you the condition of your teeth and fillings. You might ask your dentist if he can give you more information about why replacement is important - or share the findings from your x-rays or photos (if he has some) & it might help you make the decision of whether or not to proceed. There is no need to go to a specialist - general dentists are trained in the removal and replacement of restorations. As far as health risks go - they are minimal. The metallic mercury in your fillings is far less dangerous than organic mercury found for example in seafood. And the amount of time you are exposed to mercury during removal of the filling will be very minimal. If you do decide to replace - you can request a dental dam - this will minimize debris & help prevent accidental swallowing.