If you open and change an Access db (2003) with a newer version of Access (2007), would you still be able to open it again with Access 2003? +Rep for answer.
To be safe, I would save the db with the Save As ... method - to force it to save in 2003 format. An easy way to test your question is to make a backup copy of the db, open it with 2007, save normally, close, and then try to open in 2003. But even if you can open part of the db again in 2003, you might have some changes - like maybe with macros (programs) or some other subtle things from updates in Access 2007. Again, to play it safe, force save as 2003 format when in 2007. If that's not available, try asking for the 2003 version to be updated to 2007. You could also try this question in the microsoft tech support forums on the microsoft web site. More details than you can shake a stick at.