As part of the Release 12.1.3 upgrade project, we upgraded the 18.104.22.168 Apps database to 11gR2 (22.214.171.124) from 10.2.0.4.
Best practice is to seperate database upgrade or any platform migration activites from the actual R12 upgrade project. Many are tempted to use the downtime provided by the business for a R12 upgrade to schedule a 10g to 11g database upgrade as well as in some cases a platform migration or 32 bit to 64 bit conversion all in the same downtime.
From the point of view of availability that is great, but by combining all these activities into one task it is only going to increase the risk as well as complexity of the actual R12 upgrade.
In our case, we were moving from Solaris to Linux platform as well and we performed the platform migration on the 10g database itself.
We then pointed the Solaris 126.96.36.199 Apps tier to the 10g database on Linux and then performed another task prior to the R12 upgrade which converting to OATM.
After the OATM was completed, we are now upgrading the Linux 10.2.0.4 database to 188.8.131.52.