Sig will, from time to time replace a slide, however as far as I know they never re-serial number a slide. Also, Sig no longer puts serial numbers on any slide or barrel that I know of. I currently have a a 220, 225, 226, 228 and 229, and the only models with the "trifecta" (same number on barrel, slide , and frame) are older, and made in either Germany or the former West Germany. This is still the case, as my brand new 228 has the the 3 numbers.
Sig replaces the slide when there are "significant" problems with the internal workings, as the new slides are milled stainless steel. The old stamped steel slides (made in Germany) had a breech block, which was held in place by punch pins, and thereby able to be replaced.
As far as value is concerned, NRA grade 90-95 Sig 229's typically go between 550-625 with 2 magazines. Auction settings get less, and retail typically get more.
The 2 additional mags add between 30 and 70 of value depending on where they were made (italy, Germany, or USA) and wether they are market LEO. I would say 70 for german made LEO stamped with Sig Sauer logo on side, and 30 for USA made. Plus or minus $10 depending on use, wear, and spring life.