There's a huge difference between 5.3 and 6.4, so I don't think a straight-forward configuration upload will work without problems. I would not downgrade either - 5.3 won't support new APs.
Here's how to get this done:
1. Boot up the new controller, provide it appropriate LAN and Internet port IP addresses, and Default route
-> the default route should point to LAN or Internet port IP range rather than a VLAN
2. Copy the old controller's configuration manually to new controller
3. Enable teaming on new controller, and configure it to be the manager
4. Restore old controller to factory default settings
5. Provide old controller appropriate LAN and Internet port IP addresses, and same Default route as new controller
6. Enable teaming on old controller
This way the new controller will become team manager. Once old controller connects, the manager will first force a FW upgrade, and once that's done it will re-configure the member, and the team will form.
The reason to point the Default route to LAN or Internet port IP range instead of a VLAN is simple. If you point it to a VLAN IP range, you will have to create that VLAN on the member controller before teaming, and provide correct IP interface for that VLAN. Once the member joins the team, the manager will wipe the complete configuration except for the LAN and Internet port IPs and Default route, and re-configure the member. If VLANs exist, they will be re-created, but the manager cannot provide IP addressing for them.
So if your manager's Default route points to a VLAN IP range, the member will not have an IP interface in that subnet and teaming will fail with error "Invalid route".