I am assuming that you have done a lot of work already on an Arrangement and now want to change the whole arrangement globaly. Here is how I would do it:
1 - Make sure your midi notes are all quantised and of uniform length whatever their note values. ( This applies especially if you have played the notes in in real time from a keyboard which is likely that they will be uneven - so quantize the lot before doing this. If you have 'written' or dragged the notes in Step Time then it is not so critical)
2 - Select all the regions and then grab the Right hand end while holding down OPTION and SHIFT - you will be able to drag all the regions to the same uniform length.
3 - Calculate the entire length of the regions you have already written and round them off to a set number of bars. Let's say for example the regions extend from bar 1 - to bar 16. You are going to reduce the regions to exactly half their length by OPTION dragging the regions smaller. That way you will get quarter notes halving to 8th notes. ( If it was the other way round you would OPTION drag the regions larger the other way)
Hope that helps