If you decide to run live on an upgraded server for any amount of time, you have the problem of rollback - whether you upgraded in place or did a side by side.

Sometimes, you can setup replication between the servers in a side by side scenario to help with that potential rollback.

Sometimes, you can use a tool such as RedGate SQL Data Compare to get the data back to the old prod box.

My question would be, "why proceed with putting the upgraded server into production mode if the testing has not been adequately completed?".