that would depend on what U S parts were used allready. if it was a U S grip and a euro stock and you remove both of them and put on a euro thumbhole you have not added to the euro count even thow you took of one US part because now their is NO pistal grip at all to count. the easyest thing to do is look at the list and decide if you have ten parts from the list on your gun or not. most guys do the math backwards and it works out the same but for new guys I think it is clearer to count and see if their are ten parts from the list or not.
with this in mind if your gun has a US stock and a US grip , then you take them both off and add a euro thumbhole stock , you just added one euro part and that could be trouble. I hope this helps.