I think "YMMV" would apply here. My local stores have not accepted returns on opened prepaid phones in a long time now. While their return policy doesn't specifically say anything about prepaid phones, it's pretty vague and open to interpretation by the store and I've had problems with that. I was able to return an open Verizon Samsung Intensity though when it kept dropping the network; I told them I absolutely did not want the same phone - the manager begrudingly gave me a refund after talking to electronics and making sure I understood she was doing me a huge favor.. Right. When I ask them before purchase though, they tell me no returns.
So.. Up to you if you want to take the risk. Honestly, the Optimus is a great phone and I'm keeping mine as a backup even if I do plan on buying the Triumph. I do agree that return policies are getting much stricter due to customers abusing it. I'm not going to get into it but I expect Target to severely shorten and tighten up their return policy on electronics soon.
Castrol EDGE 5W-30 is my drug of choice.
Primary: iPhone 6 64GB, Galaxy Note 3, Lumia 635: 5 lines, 3x $5 Android web, 1x $2.99 T-Zones, corp discount + $10/mo loyalty discount = $93/mo tax inclusive
Secondary: iPhone 4S & LG Ultimate 2: TracFone (Verizon)
Tablet: iPad Mini Retina w/ T-Mobile 4G LTE ($10/mo for 5GB with hotspot)