[jpstewart]

I doubt it. 

For starters, the Ultra 25 chassis comes with a 400W power supply while the Ultra 40 uses a 1000W unit.  The Ultra 40 has more and larger cooling fans, too.  So you'd need to change more than just the motherboard.  Also, the Ultra 40 chassis is about 10cm deeper so things probably won't fit in the Ultra 25 chassis.  At first glance the two look quite similar, but dig a little deeper into the specs and you'll see big differences.

FWIW, you might be interested in the Sun System Handbook (not my website):
https://dogemicrosystems.ca/pub/Sun/Syst...index.html
The systems you're asking about are described in the "EOL Systems" section.