I think you either have to live in the country of issue or have another suitable reason to change.
I just had 2 mornings at CAA trying to get a type on my JAR Licence. To no avail, ended up with an EASA Licence though, this was by sheer luck as fortunately there was an English Lang examiner passing through in limbo with the rest of us.
So anyone thinking of doing what I did bear in mind.
1. English Lang 6 has to be written on your licence somewhere
2. You have to have an EASA licence, while the JAR is legally an EASA Licence they do not deal with them anymore.
3. The skill test must be conducted by an examiner approved by the CAA. There is a procedure for the examiner to follow to do this as laid out by the CAA and is a one off.
5. Must have 500 hours logged as of date of skill test