I think it's OK and there's no fiddling about with the existing grades. In the workshop module the error-banded grades (and all the other types for that matter) are scaled to the assignment grade when stored in the assessments. [This is actually not the best way of doing it, the exercise module stores grades internally out of 100 and uses the assignment grade on-the-fly when displaying them. This means you can change the assignment grade at any time which is nice.]
Your only(?) problem could be any assessments done while the grades in the error bands were incorrect (that is scaled in the range 0-100 instead of 0-50 in your case). I suspect those will be double what they should be.
You'll need to goto in the Administration page and check out several assessments to see if that pans out. In the workshop module the grade that gets displayed at the top of the assessment page is the figure that stored in the database.
In the fullest of time I want be change the Workshop module so that it stores grades in the same way as the Exercise module, but I need to think through how and where existing assignments can be preserved.
Please let me know what you find,