I think you are making this a bit more complex than it needs to be. First let me deal with the specific question that you ask "what does it mean by 'update merge with'" When you copy a file into a location which has an existing file, what happens to the existing file?
- Is the existing file effectively deleted and is replaced by the new file? This is what happens when you copy run start. The old startup file is removed and completely replaced by the content of running.
- Or is the existing file retained and the content of the new file inserted into the file? For every line in the new file if there is a corresponding line in the existing file the line is replaced. For every line in the new file if there is not a corresponding line in the existing file the line is inserted. What about lines that are in the existing file and there is not a corresponding line in the new file? The line is retained. For example if we assume that the existing file has access-list 101 and the new file does not have any access-list 100 then the result is that access-list 100 is still in the running config.
Do you have, or can you get, a copy of startup config from the old ASR920? If so you might just copy that file to running-config on the new ASR920 (and after you verify that things are running as expected then copy running to startup). But I would suggest that there are some lines in startup from the old machine that you might not want to copy into the new machine, such as:
boot system bootflash: <NEW IOS-XE>
license udi pid ASR920xx sn <NEW S/N>
My suggestion is that you get a copy of the file from the old machine, edit and remove lines that are perhaps machine dependent, and copy the result into running config of the new machine. Check its operation and then copy running to startup.
HTH
Rick
