There are threads on NES re: this exact question.
AFAIK there is no answer on that because the law does not specify and I am unaware of cases where the question has been addressed. It is similar to pinning of a collapsible stock on semi-auto rifles....what constitutes legal vs still an AW?
maybe someone else out there has a cite but I do not.
personally I would rather just purchase a 10rd mag and be done w it. However, if the mag is pinned or fixed in some manner that it cannot be modified back to 17rd, then I would personally feel OK using it...i am not giving legal advice just my personal approach.
if the mag were soldered or different follower used, something at lease quasi-permanent then I would feel fine. If its like a stupid plastic twig or very temporary fix to 10 rds, then nope.