The machine's MAC address changing won't affect anything. Active Directory isn't concerned with the link layer. Yep you can just plug it back in, and the DC will catch up. (Now on the other hand, if the DC's hard drive was encrypted with something like Bitlocker, replacing the motherboard (and thus the onboard TPM chip) would mean the end of the road for the data stored on that hard drive. But I will assume that you're not using Bitlocker. :))
There is a limit to how long a DC can be offline and still be brought back successfully, called the tombstone lifetime. It is probably 180 days for your forest though, so you are nowhere near having to worry about it if the DC was out of commission for "just" over the weekend.
But don't allow there to be multiple DHCP servers on the network. Consider removing the DHCP Server role from the machine before reconnecting it to the network.
Edit: As mfinni says, it's really more that you don't want overlapping scopes. I was assuming that your new DHCP server and your old DHCP server served the same IP address range. So instead of removing the DHCP server role completely, if you intended to bring the old DHCP server back into service, you could just modify its scope instead so that it doesn't overlap with the current one on the network.