Alas, createNew will blank out previous request data. You would then have to manually copy the elements you want into the new request. The alternative is to for createNew=false with a previous step copying the essential elements you need later. Something along the lines of:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage async="false" continueOnError="true" enabled="true" name="assignSaveMessage">
<DisplayName>assignSaveMessage</DisplayName>
<AssignVariable>
<Name>save.request.content</Name>
<Ref>request.content</Ref>
</AssignVariable>
<AssignVariable>
<Name>save.request.verb</Name>
<Ref>request.verb</Ref>
</AssignVariable>
<AssignVariable>
<Name>save.request.queryparam.foo</Name>
<Ref>request.queryparam.foo</Ref>
</AssignVariable>
[snip]
<AssignVariable>
<Name>storeIdAccessToken</Name>
<Ref>access_token</Ref>
</AssignVariable>
<IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
<AssignTo createNew="false" transport="http" type="request"/>
</AssignMessage>
You could then "restore" those values after you handle the callout. Like follows:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage async="false" continueOnError="true" enabled="true" name="assignRestoreMessage">
<DisplayName>assignRestoreMessage</DisplayName>
<AssignVariable>
<Name>request.verb</Name>
<Ref>save.request.verb</Ref>
</AssignVariable>
<AssignVariable>
<Name>request.queryparam.content</Name>
<Ref>save.request.queryparam.content</Ref>
</AssignVariable>
<AssignVariable>
<Name>request.queryparam.foo</Name>
<Ref>save.request.queryparam.foo</Ref>
</AssignVariable>
[snip]
<IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
<AssignTo createNew="false" transport="http" type="request"/>
</AssignMessage>
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