When I tried restoring the configuration, I got the error: "You cannot restore settings to a different router".
So I first needed to check what I had to change in the cfg file...
gunzip -c tomato_v128_mDA3159.cfg | tr "\0" "\n" | grep "addr=00:1" et0macaddr=00:16:B6:DA:31:59 lan_hwaddr=00:16:B6:DA:31:59 wan_hwaddr=00:16:B6:DA:31:5A wl0_hwaddr=00:16:B6:DA:31:5B
Target device (to be overwritten):
gunzip -c tomato_v128_m4AF795.cfg | tr "\0" "\n" | grep "addr=00:1" et0macaddr=00:1D:7E:4A:F7:95 lan_hwaddr=00:1D:7E:4A:F7:95 wan_hwaddr=00:1D:7E:4A:F7:96 wl0_hwaddr=00:1D:7E:4A:F7:97
Ah! It might check the MAC address! Let's try changing that...
gunzip -c tomato_v128_mDA3159.cfg | perl -pi -e \ 's/addr=00:16:B6:DA:31:59/addr=00:1D:7E:4A:F7:95/g;'\ 's/addr=00:16:B6:DA:31:5A/addr=00:1D:7E:4A:F7:96/g;'\ 's/addr=00:16:B6:DA:31:5B/addr=00:1D:7E:4A:F7:97/g;'\ | gzip -9 > tomato_v128_copy.cfg(NB: OS X didn't have a sed command that worked nicely with binary files.)
It restored the modified configuration file!
After resetting the modem and other connections that have the old MAC address in memory, I could connect to the internet again.
When upgrading to a different model router, the configuration files will not match. (it checks
boardtype, boardnum, boardflags and other settings).
In that case, you could also SSH to old router and export the configuration in an easier format:
nvram export --dump > 1.txt scp 1.txt firstname.lastname@example.org:~
On the new router, you can them import the edited file.