Is it possible to change the Access data source to SQL Server?
Yes.

When you "upsize", Access creates "linked tables".
The queries and forms on top of this don't know any difference

Some links:


https://stackoverflow.com/q/184406/27535 (SO)
http://support.microsoft.com/kb/2279654/en-us (MS KB)


Edit:

You'll need to deploy a new mdb/accdb to each client, along with a DSN (Control Panel, Admin tools, "Data Sources (ODBC)")