Upgrading to SQL Server 2008 R2 but downgrading the edition - will this cause problems?
Very few SQL language features are different between editions. 

Some language constructs (CREATE PARTITION for example and related) are additional (as opposed to different) and won't work because these are Enterprise features.

However, for day to day code, nothing special, things are almost identical.

The only thing I can recall that is different is how indexed views are used