As RogerCilio said.. it may depend on what you current plan is, and if you are on a term discount.
GENERALLY, you can change plans without penalty, etc.
But for cellphones, if you are on a term discount (2 year contract, with then a phone discount), there is a minimum $ level for the plan which must be kept. This is to ensure that the then device discount, is paid back then essentialy over the 2 year term. After that term, you can go to a NON term priced plan, etc (which is then normally $10-15 less than the same plan with term).
Sometimes the same applies for bundle term discounts for cable/internet/phone, etc as well. There may be minimums to keep that discount.
This is pretty much common practice across all the carriers. Its not just Rogers. The only way to really avoid this, is to not get into any term contracts. But will get no bundle discounts, etc... and on phones would require paying the FULL retail price of the phone up front.