So, that really only applies for spousal visas, if I understand correctly. It seems like you are doing a fiancé, so you don't actually upload documents to NVC (or, not the same amount.) Essentially, the NVC just acts like a holding point for fiance applications and doesn't actually do any paperwork (unlike for a spousal visa, when the NVC would actually require submission of financial support, civil docs, etc. (this comes later, when your fiancé is in the use, after you get married, and file for Adjustment of Status). When embassies have space/slot available, they take from the waiting list. https://travel.state.gov/content/travel/en/us-visas/immigrate/family-immigration/nonimmigrant-visa-for-a-fiance-k-1.html#5
The only way (that I know of) transferring to a different embassy, is to contact the embassy directly. So that email you have waiting to send - needs to be sent. If the deny it, then you will remain at Warsaw. If they accept, they will contact NVC, who will coordinate the transfer (it doesn't matter if it has been already given a date at Warsaw or not.)