IBM / AIX virtualisation means more than you are assuming. In the true virtualised environment a system will have a pair (or quad) Virtual I/O servers (VIO server). This effectively virtualises the network, SCSI (Fibre NPIV), and HDD.

The VIO servers are themselves LPARs on a P series frame but not of a normal type. Once the VIO servers have been created further LPARs can be created on the frame using the virtual I/O devices provided by the VIO server. This includes VIO provided paths to external disks or virtualised logical volumes on the VIO disk subsystem to provide Hard Disk Drives for LPAR use (rootvg etc).

Once the LPAR has been created it is loaded with an O/S. This can be AIX, Linux, or in special circumstances iseries (AS400). Restricting this example to AIX the LPAR can have a number of different versions of AIX up to and including AIX 7.1.

Therefore your assumption that 5.3 cannot be loaded onto an LPAR is incorrect. Equally incorrect is your assumption that you cannot purchase a new server loaded with AIX 5.3. The P series is delivered unconfigured with a number of processors, memory etc. Once installed the LPAR configuration takes place as I have indicated above. Also any application will run on an LPAR and have no knowledge of the fact that it is a virtualised environment. If your applications are not AIX 6.1 compatible then your LPARs in the new data centre will have to AIX 5.3.

You need to find out what the VIO environment is on the old servers and see if this can be replecated in the new data centre.

I hope this helps.