I don't know what the warnings are, but I'll take a stab at it.
You shouldn't need to erase the destination volume unless you
suspect that it has filesystem problems. That's generally not the
case, and the following procedure will get your backup updated
Choose your new iMac's startup disk as the source volume
Choose your backup volume as the Destination
Choose "Temporarily archive modified and deleted items" from
the presets menu
If a yellow caution triangle appears next to the "Customize
these settings" button, click it, and let me know what the warnings
are. Either way, go ahead and proceed to the next step.
Yes, go ahead and proceed to the Disk Center, click on your
destination volume, click on the Recovery HD tab, then click the
button to create the Recovery HD volume. When you go back to the
cloning task, the warning should be gone.