Changing GL Account on Service AP Invoice

When copying a Service Goods Receipt PO to an AP Invoice, and changing only the GL Account on the AP Invoice, the link between the documents is broken. The GRPO remains open, and there is no reference to base or target documents between the GRPO and the AP Invoice. This was not the case in 2005, and has recently been noticed by our customers after upgrading them to 2007. Has anyone else experienced this behavior? Any solutions?

Thanks