Depending on the speed of your WAN and the size of your Access tables, you may find that your app becomes too slow to use. If that happens, you'll need to switch to SQL Server where you have more control over how much data is pulled over the network.
The biggest problem with going from Jet/ACE to SQL Server is that in order to take advantage of SQL Server's ability to send just requested data back to your app, is that you'll need to modify your forms to restrict the data they request. It is quite common for Access apps to be built on forms bound directly to tables or unqualified queries. That defeats the entire purpose of using SQL Server. You'll need to change the RecordSources of your forms to queries with rigid selection criteria to bring down the minimum set of data possible.