Moved provisionally to ePO for a faster response.
There are a couple ways of doing it, the simplest probably being along the lines of installing the full SQL version, creating a backup of your current ePO DB, attaching that DB into the new SQL & creating the necessary SQL accounts for ePO to authenticate into it. After that is complete, go to the /core/config page and config ePO to go to it.
FYI - SQL Express 2008 R2 has a 10Gb limit, previous versions were 4Gb. Log into whatever server your SQl is on and check the size of the .mdf
I've attached a ePO DB guide I put together a while ago, once you do get your ePO DB setup you can go through it and see if any of it helps you to optimize the DB and speed up performance.
ePO DB Performance v1.1.docx 146.5 K