Are you using CR 8.0? In 8.5 you can set to either globally update subreports when opening a main report OR update a specific subreport when opening a main report.
For the setting up on a specific report, Click on the subreport. Go to Format>Format Subreport. Click the subreport tab. Check off "Re-import when opening". The current subreport will be updated when the main report is opened and refreshed.
For setting it globally, go to File>Options. Click the New Report tab. Check box "Re-import subreport when opening reports". Click OK. Any subreport will be updated when its main report is opened and refreshed.
Not absolutely sure if these same options are available in 8.0, but it's worth a check.
Is this what you are needing?
~D