Can changing iPad motherboard to older version bypass iCloud lock? I was curious to know if its possible to change the motherboard of an iPad to reset its software version? The iPad is locked to my friends late grandpa, and hence locked to his name. 

I was using it without any problems until one fine day when the iOS upgraded from 6.0 to 7.0 and after that it was asking the iCloud password of the device, which unfortunately none of us knows. Then I kept the iPad as a decoration, until today, when it struck me, that probably changing the motherboard might help to reset the software version to an earlier iOS version.

Or are there any other possible technical solution to bring back the iPad to life, instead of running around solicitors and lawyers? 

This is a genuinely bought family iPad / not stolen etc duly passed on to a family member.
For most intents and purposes, the motherboard is the iPad. The rest of it is just parts which surround the motherboard -- case, battery, screen, etc.

All of the device's storage is on the motherboard -- placing a new motherboard into the shell of another iPad will not give you access to data which was stored on the old iPad, because the storage was all on the motherboard you removed.

If your goal is simply to have a working device, you can continue, of course. But if you want to get access to the data you had on the old iPad, you'll want to look into other solutions.