Say I want to replace 1 file on my image, can I just copy it into the image folder on my server (as I do now with RevRdist), or do I have to go through the entire process of creating a load, then merging the loads?

Either method works. For people who are comfortable working on the Unix command line, copying files to the radmind server with, e.g., scp is very easy, but you might need lcksum depending if you are using checksums. For those less comfortable with the command line, the Radmind Assistant is an easier way to do the same thing.

Also, you can create an overload with the one file you want to replace. Then merge it with the overload you would like to update with the tool lmerge

This only works to replace the file. If you want to add a new file to an overload, you must copy the file to the server AND add it to the transcript. Files not listed in the transcripts are NOT used.