There really is no workaround here: vote this idea up: Allow templates to be changed after email created (which is included in Grégoire Michel's amazing list of bugs and missing features: Just do it! Marketo so-called minor missing features ).
Hi Nick Nappo,
Dan is absolutely right on this. Have you considered creating a master email template? I have dealt with a few client earlier who implemented a master template with about 40-50 modules. Each of these modules would have template sections which would cover branding and designs of all the email requirements in the instance.
Although switching templates is not possible, considering to create a master email template can save you the trouble and hassle of creating a different template for each email.