This may or may not work. Typically, each edition adds something to the DB that cannot be used by a lower edition. MS says they are required by the additional functionality, but I can't confidently vouch for that.
If you try and attach the DB to 2008 R2 Standard and it says you can't, then I would script out the DB and table schemas including PKs, IDXs, etc. The use the Export/Import Wizard to tranfer the data between versions.
There may be a better way to do this, but this is how I have done it since 7.0.
Maybe someone else has an easier idea.