Yep, unless your WAN IP address is a static one purchased from ISP, it will change 
As you say, all details are via the client.ovpn file so changing <ddns address> should work, but is dependent on where/how the ddns service works for your NAS box.
Is NAS box located on your premises / home or remotely ?
Either way, give it a try, easier to find out the hard way, 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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