Since the createOrUpdatePlan action was initially added years ago, we have added better support in createPlan with the replace parameter. Including more recently adding support for clearing an existing plan using: --options clear for replace scenarios where normal delete/create causes problems with other plans referencing the plan being replaced.
The clear option is actually a better solution to the problem we were trying to solve with createOrUpdatePlan. The biggest problem with createOrUpdatePlan is that your create plan script has to be more complicated by doing the appropriate removes before add things (stages, jobs, etc...). Instead, using clear means the create script can just be like any regular create script and it will still work in most cases.
This also means the update option on exportPlan is also being removed.
Remember, no matter how an existing plan is structurally modified (UI or CLI), Bamboo will not keep the logs and other status of previous runs (a long standing Bamboo request).