Upgrade to SQL Express 2008 R2 with 10 gig limit - Free
Stop the Add2Exchange Service
Run the installer which gives you the option of upgrading your existing instance. Select A2ESQLServer to upgrade.
this post outlines the straightforward process. No reboot is necessary.
You have to do the 32 bit upgrade, even on a 64 bit system, because from this matrix it would appear as though you cannot upgrade 32bit to 64bit, at least not directly.
At the writing of this article, you have to install the SQL2008 R2 upgrade on the same replication Server where Add2Exchange is installed. In the future, you will be able to connect to a new ore existing full edition of SQL Server on another box and communicate by TCP/IP.
Run the upgrade wizard and then accept defaults
after done, restart the Add2Exchange Service with the A2e Service account credentials.