Before you get a conversion (assuming your transit card is a good candidate), I suggest you do a few experiments to find out whether it’ll work at all.
I tried my very best to evaluate whether mine would, by getting traces of the transactions between my card and the readers in my local busses with my PM3, tried a few range experiments, decided to get my bus card converted despite the worryingly short range with the full-size card, and in the end the conversion didn’t work (you can read all about it here).
My advice is: unless your transit card rings REALLY easily on the readers, at least half an inch from the surface, don’t bother.