Is it okay to run sql 2000 in production and sql 2005 in development? Please rush with thoughts

Is it okay to run sql 2000 in production and sql 2005 express edition in test?

A system was developed using sql 2000.  It currently runs in production using sql 2000.  We are in the
process of updgrading the system, and the development system database system does not match the production system.  Does anyone have any thoughts on this?

I am leary of this environment since it could cause unnecessary issues with compatiblity.  Keep in mind that we are not upgrading the stored procedures or tables to be sql 2005 compliant.  We are just going to copy the sql 2000 stored procedures and tables to the test environment.  Make changes and then move them back to production.

I appreciate any help with this.  I do not have a database background, but I have seen issues in the past where the production and development systems are not the same or similar.  It had caused issues that were very difficult to find to say the least time consuming.

Please rush with any assistance.