Tool for globally changing inline URLs after a migration?

Our company is migrating our blog site, currently at https://blog.iwl.com/ to become part of our main website so that it will be located at https://www.iwl.com/blog.

The contractor has moved all the blogs posts to the new site, but, It turns out that many blog posts make references to other blog posts at the original site. 

So that means that once we turn off the old site, there will be many 404s as a user clicks on a link that tries to go to the old site.

I am wondering if there is a tool that recursively goes through all the source code, looks for instances of "blog.iwl.com" and changes them to "iwl.com/blog" or just "/blog" since it is now on the same site.

If this were Linux, I could just do a grep on the whole file system and fix it in a single one line command. 

Any thoughts or suggestions?