This article is from the Macintosh hardware FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
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. Daughtercard upgrades for the Powerbook 500 and Duo 280
series will be available sometime late this summer or early fall.