<p>I need to make a change to an ASP.NET web service written a couple years ago on 2.0.  I call this web service from an old 1.1 web site.  I need to make some changes to the web service, so am thinking, should I rewrite this into a WCF service and if so, will I still be able to use it from my 1.1 web site?</p>

You can make a WCF service act and behave just like a traditional 1.1 ASMX web service, but is that what you want?
I think you need to ask yourself what featires of WCF are motivating you to upgrade.
Do you want to also expose the service as a REST-ful service?  Do you need to implement message level security?
If it's just to go to the latest technology for the sake of the latest technology, I'd say stick with ASMX web services if your requirements for message and protocol security aren't that high and you're working with mostly microsoft technologies.
Writing a WCF service is regrettably more difficult than a plain-old asmx web service.