Are you trying to use the Pix to do authentication of users to surf the Internet?
This is a problem regardless of whether or not you upgrade. Prior to the upgrade, you don't have support for authentication of HTTPS or for secure-authentication of HTTP. So without the upgrade, your options are the same as 2,3, and 4 below.
You're options are:
1. To deal with the annoying popup,
2. Do not authenticate HTTPS traffic,
3. Authenticate using HTTP first so that you're already authenticated to the Pix and the SSL negotiation and form page aren't necessary.
4. Use an actual web filter product to authenticate and filter HTTP/HTTPS traffic.