I had this problem in the past when upgrading Win 95 to Win 98. What I found was that Microsoft only needed the older disk for a few seconds to confirm you had one and then carried on installing Win 98.
You only had to take the new disk out and install the old one for a minute or so before reverting to the new one.
I can see no reason why that will not be the case upgrading to Win 7 or at least I hope not because that is what I expect to be doing nearer the end of this month.
Should I upgrade to Windows 10? 8 reasons why you should upgrade to Windows 10... and 2 why you…