Why would single-use (harcoded) tokens not validate in the Message module, when the dynamic version does? In the Message module, why does the following dynamic token validate nicely [message:field-message-content:body] but the single-use (harcdoded) token does not @{message:field-message-content:body}

Basically changing [token] to @{token} doesn't work. 

Note: I know I have to create a new message and just can't change the template on the fly to back-propagate existing messages.

For reference from the module's readme.txt:


  Tokens
  
  
  "Dynamic" tokens   When defining a message type, it is possible to use Drupal tokens in any of the   message fields, in order to inject
  certain content into the field on the fly.   E.g. Entering the string
  "[current-date:short]" to the message text will   display the current
  request time instead of the token.   E.g. [message:user:mail] will be
  replaced with the message author's username   (When displaying the
  message).   If the message has fields (e.g field_node_ref), its
  contents will accessible   by the token system as well under
  [message:field_node_ref].   (For instance:
  [message:field_node_ref:title]).   This relies on "Entity token"
  module that ships with Entity API.   Enabling "Token" module is also
  recommened, as it provides more tokens   and shows a token browser in
  the message type creation page.
  "Single use" tokens   The single-use tokens are similiar to the dynamic tokens, excepet they're   being replaced by their content as
  the message is created; Meaning this   content will not get updated if
  it's reference gets changed.   E.g. "@{message:user:name}" - Will be
  replaced by the message author's name   (When creating the message).
  You can use this for example when you know the user's name is not
  going   to change, so there is no reason for re-checking all the time
  the user   name -- hardcoding it makes more sense.
  


Note: There is nothing in the error logs. Essentially when I use the dynamic token, it works and if I change it to a hardcoded token, then it doesn't. Basically changing [token] to @{token} it no longer works. I know I have to create a new message and just can't change the template too.
Provided you're using Rules to send the message, i'll suggest you to manually save the message entity after having set all required fields but right before sending the message.

Single Use token are evaluated at save time.