That's not exactly true for all cases tho, there is always a chance that hash boards from different miners are compatible with one another, I had success in swapping hash boards from different miners without having to use custom firmware or touch the EPROM, also, if the reseller knows his stuff, he would probably program the dashboard before shipping it assuming he keeps a copy of the hex from the miner before sending it.
With that being said, if so happens that the dashboards are not compatible with one another, I think using custom firmware is easier and cheaper than going the other route.