If the bass clef changes to treble clef in the music sheet, would you read the bottom treble clef as the bass clef notes or treble clef notes?
Play the notes as written. The notes on either clef are defined and fixed. They do not change.
You could write any note on any clef, given enough ledger lines. Some instruments have a tradition of switching between clefs quite often, while others have a tradition of using ledger lines instead. The rules of notation say you can switch clefs or use ledger lines. It is possible for ledger lines to become inconvenient, making the clef switch more ideal.
Your question implies that there is a difference between the treble clef notes and the bass clef notes. Well yes, and no. They represent the same set of notes but they represent them on different locations of the staff.
Since this is piano notation, the switch in clefs may indicate that you left hand will be playing notes in a range that your right hand usually plays.
You would read it as treble clef. It is common to change the clef if the notes start going high. This way you can avoid adding a lot of extra lines above the bass clef, which makes it harder to read.
It is possible (and quite likely) that in some point in the song, the clef will go back to the bass one.
To add a bit to the other answers, switching between clefs is simply a matter of notational convenience. If lower notes--typically those taken by the left hand--are higher than usual, it may be more convenient and clearer to write them in the treble clef. Here's a pretty clear example, from Mozart's C major sonata. Look starting at :29, where the left hand goes higher.
If all those notes were written in the bass clef, there would be a whole lot of ledger lines as they are above the clef. Ledger lines are harder to read, as they have fewer reference points (it's easy to confuse, say, five ledger lines with six).
In a lot of piano music, the upper staff is always in the treble clef and the lower staff is always in the bass clef. However, the point is that, regardless of the range of notes being played at the moment, the upper staff should show the part for the right hand, and the lower staff should show the part for the left hand. If the part that is to be played by one hand has to move up or down by a couple of octaves at a certain point in the music, then the sheet music can be written with a change of clef to make it easier to read.