I have created a new thread as your problem relates to RH7, not the RH8 problem that is running currently. Also I have moved it to the RH HTML forum as it is specific to that edition.
I cannot say RH did not do that but I completely fail to see how. Unlike some specialised programs, RH has no function to change file extensions and if reverting it to HTM does not put things right, then I would say something else has clobbered your project.
You say you are going to use the output files. That is not a simple task. The link below explains what you will need to do.
You will still lose various features introduced in later versions of RoboHelp. My Opening RH Projects may help you there, depending on whether other file extensions have been affected.
Do you not have any backup procedures in place?
See www.grainge.org for RoboHelp and Authoring tips
We do have a back-up procedure - copies of our projects are on a server, so if needed, we can get a restore. The project that my co-worker was working on was created in R5 and had not been opened in R7 until yesterday. I'm going to guess that the problem occurred in doing so - I'm really not sure. She is recreating the project and had planned to import the .htm files (from the output) back in. I will send her the link you provided for doing this.
Thanks so much for your help and response, Peter.
Thanks for your help today!