We have a router ( 941nd V4 ) that needs replacing, but we are new to OpenWRT so if possible we would like to simply copy the config from it. I read on the wiki that this would be compatible with a 940n V1, but for 940n we can only find the V6.1 variant.
Pretty much guaranteed. Right across the top of the wiki page is:
Devices with ≤4MB flash and/or ≤32MB ram suffer from limitations in usability, extensibility and stability of operation. Consider this when choosing a device to buy, or when deciding to flash OpenWrt on your device because it is listed as supported. See 432_warning for details.
Devices with sufficient resources are generally available in the US$20-40 range. See, for example
Past that, hardware IDs change and while close, there may be changes between models.
The configs mix very model specific settings (network settings, switch configuration, wireless device paths, LED/ GPIO configuration) with more generic ones, you can't copy them between different models (at least not without very careful checking each single setting).