Neither Arbortext nor PTC has ever guaranteed page fidelity between versions. We are in the process of upgrading from 5.2 to 6.0 and it seems like 6.0 tends to handle keeps a little differently than 5.2 did. Generally speaking, it seems like 6.0 tries to render more content onto a given page than was previously done with 5.2. We are still comparing and investigating, so your mileage may vary.
It also seems that we had some invalid lines of ACL code which Arbortext 5.2 would happily ignore and continue running successfully. Whereas 6.0 will halt and throw an error when encountering the same ACL code in the same context.