I don't have any version of Acid so you will need to discover the specific steps in that software to achieve what you want to do, but ..
When you change the key of an audio file it must be resampled. If you are not able to also ensure that the file's total playing length is not changed the tempo will be changed. You would normally try to find a setting in your software that resamples the audio file without changing its length. There are third-party apps which can do this "automatically". Perhaps one of the best is Melodyne from Celemony Software but it isn't particularly cheap!
Also, you should really only try to do this on a .wav file, rather than an .mp3.
HTH and sorry I can't be more specific about how to do this in Acid.