Unfortunately if u want to do a upgrade on ur account u must change ur plan to one u of the plans they are offering. You can contact Rogers and see what they can do tho. The stores have no access to do anything.
You only need to change your plan if you want a smartphone - if you want a Smartpick device you can keep that grandfathered 6 gig plan of yours. You can see Smartpick devices through Rogers.com - otherwise you'll need to switch to a Shareeverything plan. I would call in to see what plans they have available for you.
While that is true most with a 6GB plan will need to change either way unless somehow a rep at Rogers can keep them. As i said call in and see what plans they can offer to the OP an see what they can do for the OP.