Changed out my RB600A board to a RB800 board today. I also have the RB816 expansion board. Since they had the same port layout pretty much, I created a backup of the config and downloaded that to my pc. Then I shut it down and changed out the board in the box. When I powered it back up I found the router in winbox by mac address. I then upgraded the os to 5.11. Then I restored the config back to the router and rebooted. When the router rebooted I tried to get into the router.. so dice.. so I had to dig out my null modem cable and log into the router. Once I got into it I noticed that the Ethernet 1 and Ethernet 3 interfaces were renamed to Ethernet 20 and Ethernet 21. Why would Ethernet 1 and Ethernet 3 disappear like that? Is that a bug?

As far as I know (and experienced), .backup files (binary backups) are only 100% compatible if you use the exact same device (by that I mean restoring the backup to exactly the same router you took it from) and about 95% compatible within the same series of routers (using a backup from a RB750 on another). Putting a RB450 Backup on a RB750, or in your case a RB600A binary backup on a RB800 mostly doesn't work at all, forcing you to netinstall, or produces all kinds of general weirdness and problems.

If you absolutely have to change the board family, try to restore from a text file configuration backup (/export).