The UT50 is not a direct replacement, really - no wifi adapter, no filter, only 2 HDMI. The ST50 is an equivalent. Problem is, Best Buy only carries the 50" ST50 online...the only size they carry in-store is the 55". I am not sure if it's orderable through the store's system or through Bestbuy.com only. If the latter is the case, then they won't be able to give it to you as a replacement. The Samsung PN51E550 is comparable and likely a good option, and even comes with two pairs of glasses in the box. The "real black" filter on the 550 model is pretty bunk though, and the picture quality probably isn't as good as your ST30. It might be pretty hard to argue that two models arent comparable if they suggest it. Worst case, the Samsung sells for 999, and that's the credit you'd receive.
If you stand your ground, and know your specs, then you'll be good. It's not so much the fact that Best Buy wants to screw you over, it's that either the comparable SKU finder app they use recommends something weird, or that the employee you're dealing with isn't aware of the intricacies of your older model. If it is determined that Best Buy doesn't carry anything comparable to what you bought, then you get the full amount of your original purchase price back on a gift card. A few months ago I had to do this for a customer with a failed Samsung B750 CCFL LCD. Since we didn't have anything like his TV anymore, he was able to buy a new LED TV, warranty, and a Bose Cinemate with the gift card we gave him.
ht Panasonic TC-P60ZT60, Monitor Audio: Silver RX6, RX Centre, Radius 90HD; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D
2ch Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Monitor Audio Silver RX1, REL T3, Apple TV, Peachtree Audio DAC•iT
lr Panasonic 50ST60, Sony BDP-S5100