For the record, the 5K iMac CPU is socketed, not soldered, so it can be "field upgraded". However, such a procedure is not something an Apple Store Genius Bar is authorized to do and I imagine any Apple Certified repair shop is prohibited from doing so, as well.
So you could do it yourself or find a general repair shop that could do it, however as noted this would void your warranty if that replacement CPU was present when you filed a claim and I expect there are ways for Apple to identify the machine was opened which in itself may void the warranty if there is not documentation the opening was done by Apple or Apple-certified shop.
Also, as ridiculous as most Apple upgrade prices are, I think the i5 to i7 (or base i5 to better i5) upgrade prices are likely less than what you would pay for the CPU at retail (factoring in the resale), much less pay for the CPU and the upgrade labor from a shop.