Can I rewrite a HTTPS header using an iRule

It depends.

 

If you see it clear text (i.e. with an http CONNECT method in front of a proxy) it is easy to modify the URI on the fly.

 

If you want to change it i.e. with a http GET request you will need to touch the host header in the request. The URI remains the same. But in this case the traffic is very likely encrypted and you need to apply SSL termination and reencryption.