Q.Shaping And Growing Leylandii Trees
I have a couple of leylandii trees which I use as screening. They are approximately 8 ft tall and 4 years old. I have trimmed them each year and they are now the typical pyramid shape. My question is if I trim off the lower branches completely to a standard tree shape will the remaining upper branches continue to grow upwards with the trunk over the fence line so I no longer have screening? My intention would be to keep the hedge at around 8ft with yearly trimming.

Certified GKH Gardening Expert
These trees will enjoy a yearly pruning! They handle frequent trimming and will benefit from it. As long as you maintain the structure, it will continue to fill out. Letting large, bare trunks form will make it less able to be pruned. Cutting into branches with bare bark will leave bare spots that will not fill out.
These articles will help:
https://www.gardeningknowhow.com/ornamental/trees/leyland-cypress/pruning-leyland-cypress.htm
https://www.gardeningknowhow.com/ornamental/shrubs/shgen/care-of-landscape-hedges.htm