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 60" ZT60, Monitor Audio: Silver RX6, RX Centre, RX1; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D, Panamax M-5100PM
2ch Sony 32" W650A, Sonus faber Toy Monitor, REL T3, Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Apple TV, Peachtree Audio DAC•iT, Panamax MR4300
pc Monitor Audio Radius 90HD, Audioengine D1, FiiO A1
+ Sony 65" XBRX850B, Sony BDP-S5100