I have one Proxmox Cluster and i create VMs with Machine type i440fx. Can i change the Machine type for EXISTING VMs to Q35 or i need to create new VMs ?
Not always (as you also imply). I just tested it on a trivial Debian Server VM - and although it booted correctly after changing to q35 from i440fx, it lost it's NW connectivity + GA. So anything is possible!