I'm afraid that what you're asking is not possible, there's no cryptographic way of making a 1024 bit sign a csr for a 2048 bit, and it's not desirable either.

You don't really have to go up to 2048 bit if your server doesn't allow you to, there's plenty of companies that will sell you a 1024 bit certificate, or you can even ask your current company for a 1024 bit certificate or else you'll find someone else.

Hope the answer gives you some insight in the matter!

One of the features of the Certificate Authority system is that things change. Roots expire, new roots come into existence, certificates are revoked, roots go bankrupt after compromises. An update mechanism really needs to be in place to handle these changes.

There is a third option available, and that is to use a different CA that offers 2K certificates signed with an authority that was around 10 years ago. That isn't Verisign, but may be someone like Thawte.

If that proves unworkable, you'll need to go with a blend solution. Provide a test site with the new CA you picked for clients to validate against, and offer plenty of notice that the certificate will change and will break up to 25% of clients. Build as much update documentation as you can think of, and build more as clients run into issues. Stress that updating CA information is something that all systems need to do once in a while, so documenting this is a good thing in the long run; it's a best-practice.