Let me edit this to be completely clear. Static A records and static IP addresses are two completely unrelated things. It is normal to have machines with static IPs have dynamic DNS A records. You should not change this unless you have a compelling reason.
Is there a way to change an automatic DNS record to a staic one?
So that it doesn't get deleted by scavenging.
You shouldn't have to do this. If legitimate records are being scavenged, then you have configured scavenging incorrectly. You should read this link. It's for 2003, but the concepts about scavenging and refresh intervals are unchanged.
This server does run a DHCP service as well, which is fine, but I want to change some dynamically assigned ip address to static (as they are static on the client end. e.g. servers)
If the IPs are static on the servers, then there's nothing more you need to do. A static IP is a static IP.
I think part of your misconception is that you misunderstand how DNS records are updated. Yes, you can have DHCP update your DNS records using the DnsUpdateProxy group, but this is bad bad bad and it's not the standard configuration, so unless you've explicitly configured this (which I'm betting you haven't), then that's not how it's being done.
By default, an computer will send a DDNS record update to its primary DNS server when the adapter received an IP address and each day after that. The adapter registers itself and the kind of record that is created is a dynamic one. If it is being scavenged, then either your scavenging window is too small or you need to shorten the interval at which computers register themselves.
Having a static IP and having a static A record are two completely different things. In almost all cases, you don't want to create static A records for hosts that self-register. You only want to do this for hosts that don't, like *nix servers on a DNS server that only accepts trusted updates from AD clients or for something like a secondary interface with a dedicated IP that's hosting a website.