When I upgraded a while ago from CS2 to CS5, many of the files I'd finished and sent to the printers were significantly altered when opened in CS5 (orphans and widows were created, generally the spacing of words in the paragraphs was altered). Can anyone let me know how "safe" it is to open a CS5 file in CS5.5? I expect to have to make very minor corrections to a number of files and don't want to have to typeset everything all over again from scratch because of the upgrade.
There were huge differences in the text engines between CS2 and CS5, but CS5.5 uses the same engine as CS5 so you should be fine. If you're worried, Export from CS5 to .idml and open that in CS5.5. That will force any reflow to happen when you open the file rather than waiting until you click in a story.
Keep in mind that the only way to go back from CS5.5 to CS5 is to export to .idml. The .indd file is not backwards compatible.