Linking to an existing ASP site (IIS/Windows)

I hope this is the right forum for a “how to” question, to which I have not been able to find an answer online. (Please don’t say I should be hosting on Linux as that is not an option ?? )

I have a large and well-established ASP site that I am hoping to retire gradually, all of whose internal links are relative. I intend to create a new WP site that will be the first port of call, with a link “if you want the old site, link“. I can install WP in a wordpress subdirectory and set it up so that calls to the default page are redirected to the new site.

But I’m concerned that if anyone follows a link outside the subdirectory (to a root file like index.asp or files in other directories at the same logical level as wordpress) it may be caught and redirected to 404.

Short of trying it out and risking screwing up, can any experts shed light on this for me please before I embark on development?

TIA