My thinking is before doing anything about the O2's...

You mentioned that the plugs were changed and they were white...maybe the ECU had gotten used to these plugs. Now with new plugs the ECU doesn't know what to do fast enough to adjust and throws a code. Take both battery cables loose for 10-15 minutes (WITHOUT EITHER BEING ATTACHED TO THE BATTERY) and then touch them together to completely re-set the ECU. Make sure you have your radio code because you'll have to reprogram it as well as setting your clock. If the above scares you, find out which fuse is the ECU's (It's one of the ones in a larger coupler in the fuse box that's triangular under the hood...at least on my S70, near the driver's side windshield...your owner's manual should tell you) and pull it for 10-15 minutes. Hope this does it for you...if you get a CEL again, then I would check out the O2 sensors...usually the front one goes first. Hopefully my method will clear your codes and reset the ECU for the new plugs. Good luck
--
1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon