Prepaid Convert to Mobile Hotspot plan

I currently have a long standing prepaid plan on SIM card which I used to use in a mobile handset.  I no longer have any use for that.  It currently bills me $3/month to maintain that service.  There is a remaining pre-pay balance on the account of presently around $38.  It will be closer to $30 by the time I need to do what follows…….

In a couple of months I'll need a PrePay data only plan and SIM for a mobile WiFi hotspot that I already own.  I guess I'll choose the $50x1 month plan for that.  Ideally, I'd use the same SIM and that residual balance.

I went partway through the process to change my plan online but I didn’t finish it as I’m not clear about how that plays out. 

Do I understand it correctly?

1. the SIM is the right size/shape to fit the MiFi, but is many years old.  Is there any probability it won't work if the plan on it is altered?

2. If I change my plan on that SIM/account/number (either from the next billing date, or from "today" if I wait till I need it to start) to a data only plan for $50 x 1 month then I will only need to top up the PrePay balance enough that there is $50 in there, to cover the monthly fee i.e. by about $20.  And again a month later if I want to extend service.

Thanks for any insight!