Is it the case that if a price change of a plan is implemented (up or down) then existing subscribers must unsubscribe from the existing plan and then upgrade to a new (price adjusted) plan?
Yes.
If that is the case can I ask you to look at how the workflow of that change could be made easier for the subscribing member? For instance, can you imagine a scenario where an 'upgrade' to the new plan would deactivate the old plan automatically?
I believe you can also cancel their recurring profiles directly in your own PayPal account and then change the price of the plan so they have to renew manually at the new price. If not best case for a price change where recurring subscribers are present is to copy the plan. change the price in the copy. Then disable renewals and new subscriptions to the old plan. When they go to renew it'll cancel the subscription and they'll need to upgrade to the new plan with the adjusted price.
I'm assumming that if the plan is de-activiated then the existing recurring payment arrangements would be deactiviated as well.
No, it just becomes unpublished. It needs to actually expire. You should be able to cancel their recurring profiles within your PayPal account though.