Its been a long time since I’ve even heard HighMat mentioned. I never used the format.
Does it play in VLC media player? If so, you might be able to convert to another format in it. Open VLC, then click Media–>Convert/Save.
Have you tried any of the conversion programs that are normally used to make DVD-video? Examples would be AVStoDVD or ConvertXtoDVD (this second one is not free).
I might try one of the gui’s for ffmpeg also, like Video To Video.