you can open old project files in fl8 as they are..... but i dont think it can be done in reverse
as long as your samples are all still in the same place all will be ok
but having said that, kamas right, saving projects as zips is a good idea, losing samples from a track cos you have moved them is a fookin pain
personally i dont save zips, i save the track in a folder of its own, and then "export data files" and it saves all the samples in the folder with the flp....... the reason for this is because i save a new flp file nearly every time a save my tune.... cos some of my tunes have so many flps, if they were all zips it would take up too much room