Answer: What you describe is actually a common practice with Giant Bird of Paradise. Older plants such as yours are sometimes trimmed in this fashion to make them look like trees. This type of pruning doesn't seem to hurt the plants, but instead of tropical looking shrubs, you'll have tropical looking trees, with the blooms high up in the canopy.
It's too late to change the appearance of your plants. However, Bird of Paradise tends to send up shoots from the roots so eventually you'll have a cluster of new plants at the bases of your trees. If you allow them to grow and become shrubby, and you don't like the bare trunks of your originals, you can cut the original plants down to ground level.
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