If you specified a title company in the contract you wrote, then decided to use a different title company, then yes, you are supposed to do an amendment to change the terms of the contract (ie which title company is being used). It is very typical when you are purchasing a short sale to use the same title company for both the buyer and the seller. The majority of the time, we tend to go with the title company that the seller is using in this case because they are the ones who have actually been helping to work the short sale. The banks require preliminary HUD documents to be created by a title company and sent to the bank so they can determine their "net" after the short sale. Quite often the form has to be redone multiple times by the title company on the sellers side. As a result, they end up being very familiar with the contract and what the bank wants. This is one case where the agents have learned from experience that having the same title company on both sides helps to ensure that the home will close more smoothly
When working with reputable title companies I find that their charges are very similar. While there are many title companies out there, in reality there are not a lot of underwriters that actually ensure the title work so often you are using the same underwriter, even when you are using a different title company and getting the same protection.
i hope this helps you to feel more comfortable about the situation. If you have any further questions, I would suggest you talk to your Realtor and/or to the title company that you used for the sale