You speak more Icelandic than me!
It sounded like more fun to create a program that did this than to Google whether one existed, so I did. It's attached, in a .zip file, and will run on Windows with Java. However, I didn't have time to polish it much and must head to sleep now, so a few notes:
*The utility is case-sensitive.
*Make sure you tab off of the "New" column before hitting "Replace characters in folder...". Otherwise the "New" value will be recorded as a null. This is the one thing you will get a warning about. Similarly, you should tab off a cell before adding a new row, or you'll have to re-enter the value.
*Be careful about using a character in both "Current" and "New". For example, if you replaced "h" with "t" and then "t" with "x", then all of your original t's as well as all of your original h's will become x's. The night is too short for more sophistication.
*Do not use this utility except on Windows. It will cause file name havoc if you do.
*Do not use this utility with files with more than or fewer than three characters in their file extension.
And most importantly,
*Back up all the files first. The utility has "Quick and dirty" in its name in part because it's only been tested in simple situations enough to say, "It seems to work". The most likely, but possibly not the only, side effect of it not working would be clobbered filenames.
If it turns out there isn't a drastically better utility out there I might smooth out the edges a bit in the future.
Likely my last post on CFC before Christmas, so to all a merry Christmas and to all a good night.