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Unfortunately no, you can't switch the template an email is based on.
You should upvote this idea to encourage Marketo to develop this functionality: Allow templates to be changed after email created
To support your requirements, though, you can use nested default programs to house each new email and set every person who received a past version of the emails to skip through the send and not receive them. The advanced drip nurture program template which you can import from the Marketo program template library has this type of functionality built in if you'd like to see how it'd work.