The majority of the parameters in the Bitcoin Core config file can be changed during or after IBD (Initial Blockchain Download) with no complications.
However, there are some parameters in the config file where it is important that you set them correctly prior to IBD. If you don't you will either have to restart IBD or it will greatly increase the time for IBD to complete.
Some parameters that need to be set correctly prior to IBD are testnet, regtest, prune and txindex as discussed here. Are there any others?