If you will have users who are opening the db with an older Access version,
it may cause problems to open the db in Access 2007. Access 2007 knows how
to update references from older Access versions, but once it does, the older
versions will probably not know how to revert the references to the older
version.
If you will have mixed Access versions being used, I would make a copy of
the db for each user. This requires you to split the db into a front-end and
a back-end, which is good practice for any multi-user db anyway. The backend
db stays in the older format, while each user has their own copy of the
frontend. The distribution version of the front end should be set for the
oldest Access version in use.