Hello Kim,
following the requirements for upgrade to IOS XR you can use the following document:
http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.8/migration/user/guide/up38upinst.html
in short you need:
PRP2 with 2GB Ram (minimum) as route processors
you may need to upgrade memory in all linecards, be aware that not all linecards are supported and that you need to upgrade firmware first in order to have the linecards use the new memory.
Typically 1GB ram on linecards.
http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.3/upgrade/guide/up33dual.html#wpxref1135530
then you need a recent IOS image (12.0(33)SY roughly) for two reasons:
the IOS image allows you to upgrade the various firmware types of linecards and supervisors to minimum required versions before conversion starts.
the IOS configuration file must include the configuration of IP address on one management ethernet per RP, in order to be able to perform the FTP process from rommonitor. This was not explained well when we did the first one.
Hope to help
Giuseppe