Need to change post template retroactivly

For example
If I have

'template'     => array(
			array(
				'core/image',
				array(
					'align' => 'left',
				),
			),
			array(
				'core/heading',
				array(
					'placeholder' => 'Add Author...',
				),
			),
			array(
				'core/paragraph',
				array(
					'placeholder' => 'Add Description...',
				),
			),
		),

and a year down the road I need to remove the final paragraph, then change the placeholder attribute of the heading block. How do I go about doing this in a way that changes it for all previous posts but still preserves their content without going back through every single post and manually making these changes?

Making the changes in the post template does work going forwards but there doesn’t seem to be a way to change layout order and included blocks going backwards.

Am I missing something, Is a better templating system being devised that allows retroactive changes to the page?