Let's say You buy a BMW car in an official BMW shop. Would it be wrong to alter it/ repair things in Your local car repair shop, which has no affiliation with BMW at all?

In Your case You indeed get a copy, but not for redistribution or personal use, only to enhance it according to the owners requirements.

What might happen is, that the original vendor won't support the modified version anymore, but that's not Your problem in this case. You should at least tell the hypothetical client about this issue.