Don't solder anything yet. Try asking Apple first. Call 1-800-SOS-APPL or whatever their phone number is. Talk to the first guy, who will probably deny the repair, then ask to speak to the supervisor. The supervisor will usually approve the repair, and she will give you some numbers to write down. Take the eMac and those numbers to the Genius bar and they will perform the repair. The motherboard will be shipped in from whatever store has a spare motherboard lying around.
I accidentally upgraded one of my eMacs this way. It's the one I use in the warehouse as my main Mac. It was really a 1.25 GHz with bad caps (I did not know this at the time, all I knew is that it had bad caps) - but someone before me had taken and switched the CD doors with a top of the line 1.42 GHz model. So it was accepted for a board replacement and they used a 1.42 GHz board.
When I booted it up it only had an 80 GB hard drive (but it did have a Superdrive) - and I learned later that 1.42 GHz eMacs didn't suffer from the bad caps, so I deduced that it must have really been a 1.25 GHz SD model. Oh well!
I accidentally upgraded one of my eMacs this way. It's the one I use in the warehouse as my main Mac. It was really a 1.25 GHz with bad caps (I did not know this at the time, all I knew is that it had bad caps) - but someone before me had taken and switched the CD doors with a top of the line 1.42 GHz model. So it was accepted for a board replacement and they used a 1.42 GHz board.
When I booted it up it only had an 80 GB hard drive (but it did have a Superdrive) - and I learned later that 1.42 GHz eMacs didn't suffer from the bad caps, so I deduced that it must have really been a 1.25 GHz SD model. Oh well!