Once something becomes a DDS your only option is to go through and repair the quality loss manually unless you happen to have the original texture backed up somewhere. With other formats as itch mentioned (though he left out bmp) they don't see the compression loss on the actual texture, only from what the game does real-time as an effort to adjust performance of the game. The original texture when left in alternative format to DDS remains as it was originally unless you resize it. As a rule though, up-scaling a texture has no impact what so ever on quality of it, at least until you go through and enhance it after an up-scaling. If a texture starts out as 256x and is raised to 1024, it is larger but just as blurred as the original one is.. The truth is that it is quite easy to down-scale a texture if you so desire to improve performance, but doing the reverse pretty much leaves you starting over replacing details that were lost when it was scaled down by the original authors, so if you're going to scale up on a texture be prepared to invest a great bit of time in restoring the detail to the texture that was lost in the original scale.
Even though both dds and jpeg have compression capabilities, when set to 0% compression on jpeg it retains the original quality when converting to that format, dds even when set to 0% compression tends to lose some quality in the process of the conversion to that format, on top of not having much effect on file size. Alternative format options including bmp and png do not have the quality losses, but bmp is by far the largest format that I have experienced, thus I use png instead, it doesnt screw up the quality of the end-product in the formatting and is a relatively decent balance when it comes to file size.
As to the author of the topic, please try to keep your posts together, you had 3 in a row, one was content update so that one we'd let slide, but the other two were question-oriented and could have been done in the same post, in the future use the edit button found on the bottom right of your post for things like that.