Can you describe the task chain and what you want to change and what needs to be protected? This might allow us to help more?
Do you have the true relationships in the plan (i.e. this can't start until this has finished) rather than time relationships (I'd like to do this and then move onto this)? If you've got the former sort then the plan can tell you the true consequences of change. The latter usually ends up being a pain in the backside as people change their minds about the order they'd like to do things