SIM card should not need to be changed to switch from prepaid to postpaid, but make sure you do it over the phone or at a corporate store. Dealers may try to change your SIM in order to get credit for a new activation.
You may need to replace your SIM to change your phone number. That used to be a requirement for Wind (and Mobilicity) but I don't know if that's still the case.
It's the same as it always was; HSPA+. Some Android phones report that as '4G' while others report it as 'H+'. Either something on your phone changed or one of the towers changed its broadcast info, but nothing has changed on Wind's side.
What changed was a new firmware update if he is using a Galaxy S III from Samsung. (other android phones probably are the same on Wind)
Wind was at 4.1.1 and just very recently a firmware update rolled in and it changed to 4.1.2. The H and H+ went to 4G. Hamilton doesn't seem to have any H+ cells yet but my recent trip to Niagara Falls and Toronto had it kick in to H+ (at the previous firmware) and data was much faster. :-)
Not sure what it reports on a faster 4G network... 4G+?