[Brian]

The original poster was describing an action with a start date of today, and due date in a few days. They wanted to bump the start date but leave the due date alone. My (badly phrased) question was directed at them, really. Seems like an action with a due date but no start date would be flexible in the way they were looking for.

I've added an action for this script that does this to my someday/maybe list, but it ended up being too tricky for that afternoon.

The original script just says "do some stuff to my selected action". If the selection changes once the script is done, there's no problem.

For the repetition-friendly version, the simplest workflow is
complete the original action (to generate the un-modified next task)
uncomplete the original action
modify the original to defer the dates

Unfortunately the simple workflow falls down in Available and Remaining view, since the completed action disappears and your selection changes.
Need to rework the script to gets the ID of the selected action(s) and use them to keep track of what to work on, instead.