For those who are interested – I got the 16:9 clips from Liquid Edition 5.62 to work with Particle Illusion 3. It turned out to be simpler than everyone seems to have thought. I exported either an AVI or a fused sequence into PI 3 (either worked), did the effect and rendering work on the clip (which has now been changed in the process exporting from LE 5.62 to PI 3 from 16:9 to 4:3 which precipitated my original question), reimported the finished PI 3 clip back into LE 5.62. I put the new 4:3 clip with PI 3 effect on it on the LE timeline, went into the new clips properties and changed the aspect ratio back to 16:9. The clip re renders itself back into 16:9 with all proportions and perspectives exactly correct. The finished clip with the effect seamlessly works with the original. I’ve tested it with several clips and they have all worked. Happy ending.
Thanks to all for their comments and help. I learned quite a bit in the process.