515 running V5R4.. possible to create LPAR running V7R1 as well?

I'm not much of a systems or hardware guy, so I thought I'd ask here.

I'm working with IBM on upgrading from V5R4 to V7R1, but I'd also like
to keep the ability to compile down to V5R2.

I was chatting with my IBM rep who thought that it might be possible
to LPAR my 515 and put V7R1 on a secondary LPAR for development and
testing.

When I inquired with IBM they told me I would need a switchable
IOP/IOA combo and an external tape device. This is greek to me. :)

I am assuming this is required. If I wanted to partition my 515,
would that mean they'd both have to be running the same OS version?

Thanks!