@flyjet
Dude, I am in a similar boat as you. Except that I have a Canadian RT which has a lifetime validity. I did a bit of asking around and this is what I got to know
1. Your RTR (C) can be revalidated without having to give the WPC exam as long as your foreign RT is valid.
2. You can get it renewed if you have 30 hours of flying in the last 3 years
3. If neither of the above, then you need to give the WPC exam again!. Mind you, you will also need to renew your foreign RT because if that expires then what will you convert?
Hope this helps.
(PS: I am no authority on converting RT licenses. So please don't quote me or take my word as the final word)