After a week I believe the new SIM will be disable. Sooner or later you will have to switch. Visible is at some point going to shut down the old system that you are on, if you wait until after you might not get the current $5 valued member discount and you will lose service. If you have the card why not switch, if you have any issues it would be better to get it taken care of now instead of waiting until D-Day when 10's of thousands switch and customer service is inundated with issues they are trying to fix.
I made the switch 6 weeks ago when the offer first came out, I had no issues other than I didn't have data, my fault as I didn't realize the mobile data button toggled off during the card switch or activation. Took me some time to figure it out but it went smooth otherwise and I had slightly improved speeds. You may have slower speeds but only way to find out is activate the new card. If data is super slow it has been recommended to reset the network settings and restart the phone. If it persists contact customer service and have the SIM reset or re-provisioned. Good luck!