converting Rega to balanced output?

Elizabeth, best I can make out from your post, that scenario will hum. The problem is that the stock arm cable uses the minus output of the cartridge as a shield, so if that is used on pin 3 of the XLR you will have a substantial hum.

Its unfortunate that the arm does not have a 5-pin connector or else this would be really easy. But it does have the little circuit board, so it is possible.a

'I have done this operation a number of times' (Atmasphere). Well the best upgrade for the Rega is the rewiring of the mentioned part (from the collar to the pre). I have never seen such a worthless cable before.
This is, I assume, the reason why Atmasphere rewired this arm also for his friends(?). On ebay.uk you can get all the needed parts, even the special made Din 5 connector
so you can use any phonocable you like. If you are scilful you can rewire the arm all the way from the cart 'pins' to the phono-pre. The construction of Rega is such that you
have easy access to the armwand. The balanced connectors have only sense if you have a really balanced phono-pre. You should check the thread(s) about this issue.

Regards,

Addition, If one intend to solder the Din 5 connector in
the Rega collar one should or may use the cart clips for
the purpose. First solder the clips on the inside wire
and then 'slide in' the clips on the Din connctors and solder
them thereafter. The small connectors with ditto holes are
not easy to solder.

Regards,

Sweet! Thanks for the tip.