Suppose that as a non-US person, I have United Silver status. I thus have >25K PQM. However, what happens if I don't have $3K PQD but I change my address to a US address?
Do I lose the status?
If your address of record is in the US then you need to meet the qualifying rules for US based members (which requires qualifying miles and purchases). It doesn't matter what your citizenship is, only where you reside.
You will not lose the elite status that you earned from traveling in 2014, but your 2015 qualification requirements will be based on your new address.
You can always ask if they will waive at least part of the PQD requirement since you only resided in the US half the year. Or you could leave your mailing address back in your home country, as you don't really need the member card or bag tags to prove you are qualified, it normally appears automatically on your boarding passes.