I think I've got it now. If I miss the mark. . .sorry.
The Cupramine treatment is over a 14 day period. There would be no reason to exchange any water during that time so long as:
1) Water chemistry remains excellent (no ammonia, no nitrites, etc.)
2) Build up of organics isn't happening.
3) Nitrates don't go too high for the specimen you are treating.
In general, water changes during a treatment are done only on an 'as-needed' basis, with the above three items being the most reasons to do one (or more).
At the conclusion of the treatment, the water should be exchanged to remove half of the medication, at least, and then fresh carbon used to extract the remaining. At this time a water exchange would help this. Then, you monitor the fish in quarantine to verify that your treatment was a success!
:rollface:
