50 minutes is normally enough and even allows for small delays. This transfer can be done in about 25 minutes from train to train using the RER if you already have a ticket for the RER (or if you know how the ticket machines work and there's no queue) and you know your way around the stations.
If you're using a TGV or Thays to Paris Nord, then buy an RER ticket from the bar.
How to cancel or change a reservation depends on where you bought it.
Thanks, Good to know.
I’ve worked out how to cancel my existing reservations in my Eurail account.
This RER ticket is simply a local metro ticket for the inner zone: Paris-inside the boulevard periferique-also valid on RER. DO use RER=much faster, on the far left side of gare du Nord. Ticket is 1,90-only a very few machines take cash still -and mostly then have no change for 2€. Also only a very few machines for now still sell paper tickets (with old-style magn.stripe-these are announced many times to be fased out). Keep ticket-you also need it to pass gate at way-out on RER
Those ´sold´ in the bar are often non-existent, sold out/epuisëe, or turn out to be used/outdated.
As I used €* the other day from Lon; there are now-long live this brexit thing!! also spot checks by customs (=NOT passports, but illegal imported goods, it is also forbidden to take any dairy/milk/meat etc into EU-even that ch+Ham sarny)) on the way out