When browsing the report in design view, you can reach the properties of both the subreport control and the subreport itself. Find the subreport control, where you in the "other" tab, will find the subreport control name (lets, for the sample say its "MySubRptName". Now, in the reports on open event try something like this
[tt]if <your condition> then
me.controls("MySubRptName").sourceobject = "rpt1"
else
me.controls("MySubRptName").sourceobject = "rpt2"
end if[/tt]
Roy-Vidar