WHAT MACS WILL BE UPGRADEABLE TO THE POWERPC?  (3.1)
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       Apple will provide logic board replacements for all Centris, 
  Quadra and WorkGroup Server models except the Quadra 700, 900 and 
  950.  There should also be logic board replacements for the IIvx, 
  IIvi, LC 475, 520, 550, and 575, Performa 475, 476, 550, and 600.  
  Upgraded AV Macs will lose their special video capabilities, at 
  least initially.  Apple will also make available PowerPC processor 
  upgrade boards that fit into the PDS slots of the Quadra 605, 610, 
  650, 700, 800, 900, and 950 as well as the Centris 610 and 650 and 
  the Performa 475 and 476.  Macs upgraded via a PDS card run at twice 
  the speed of the system clock.  Thus a 25 MHz 68040 Mac with a PDS 
  accelerator will become a 50 MHz PowerMac.  You'll be able to 
  disable the accelerator if you have old software that's not PowerPC 
  compatible or that just plain runs faster on the older hardware.  
  68030 desktop Macs with expansion slots will be upgradeable via 
  third party accelerator cards.  It is not yet known whether any 
  upgrade will be available for Powerbooks.