Hi,
prior performing any upgrade always refer to official upgrade guide:
also check:
VMware KB: Upgrading to vCenter Server 5.5 best practices
Need your advise, if this way the setup would be stable, performance data will be retained and show up on this new vCenter 5.5
All of your performance data, tasks, events should be retained ...
Prior to upgrade backup your vCenter database and your SSL certificates or at least take snapshot of your vCenter server.
From my own experience I cannot tell you that keeping the old database is always flawless process it depends on size and health of your database and much other things ...(I am talking about upgrade from 4.1 to 5.1 release)
but things always change we are now in front of the transition to 5.5 release and I am sure that despite previous practice we will again continue with the "old" one.
Specially I must mention Linked Mode with which we have a quite hard time to get it working again...
For some experience with upgrade to 5.5 release see this great source:
http://www.derekseaman.com/2013/10/vsphere-5-5-install-pt-3-upgrading.html
(don't be scared of that picture its not that bad...;-)