I upgraded my late 2015 to an SSD about a week after I bought it. Actually a lot easier than I thought. Especially with the right tool kit and new adhesive strip kit off eBay for $10.
Check out iFixit as well, as they sell specific parts for such an operation.
I'm happy with my 2015 iMac and how I have everything setup, so I'd rather not risk voiding my warranty. Perhaps after its expiration I may take the plunge but for now everything great.