I'm really scratching my head trying to understand how upgrading to full frame with just one lens is a duplicate of Is there ever a right time to upgrade to full frame camera body?
Sure, on the surface they're both kind of about whether someone should get a full frame camera. We have plenty of other similar questions that provide specifics regarding what aspects the OP is considering when trying to make such a choice, and those aren't generally marked as duplicates of Is there ever a right time to upgrade to full frame camera body? , which seems to be primarily, if not exclusively, about product cycles, prices, and when is the best time in terms of a camera's product life to buy one. In other words, it is about how to buy the same camera (or its successor in a product line) at the lowest price.
Upgrading to full frame with just one lens isn't remotely about product cycles, when camera bodies go on sale, etc. It's more about "How useful will a full frame camera be if my only FF lens is a 50mm f/1.8?" It seems to me this is a totally different question, although it was perhaps not asked as well as it could have been.
There may well be other questions here that are closer to upgrading to full frame with just one lens than the one marked as a duplicate. It may well be that the actual question the OP asked, though not very related at all to the one marked as a duplicate, may be primarily opinion based.
I'd really like to know what the thought process was of those who voted to close upgrading to full frame with just one lens as a duplicate of Is there ever a right time to upgrade to full frame camera body?