You can keep your current month-to-month plan. Switch to a Bring your own device plan, or purchase a new phone anytime you wish.
Your monthly bill is not directly related to your device balance so you will not see any change in that regard.
Depending what plan you currently have, you may benefit keeping it or switching to a BYOD. (You will have to research the pros and cons. (Data/Min etc)
If you want the latest bells and whistles phone. Unless you’re purchasing it outright (full cost), you’ll be under a new contract/term.
More explanation on Device balance
To copy language from the link posted by WestCoaster ...
"Your Device Balance is the cost to upgrade or renew your device before the end of your service term. The Device Balance is based on the discount you originally received for your device and decreases every month over the length of your service term. At the end of your service term, the Device Balance is $0."
The language used is pretty deceiving. To call it a discount is disingenuous because it is more of a loan that is paid back over the two years. If it were truly a discount you wouldn't have to pay it back. Likewise it is not actually the cost to upgrade or renew - it is the cost to payout the "loan". And taken literally you ought to be able to get a new phone for $0 at the end of the 2 year term - that is exactly what it says - the cost to upgrade or renew is $0 at the end of your term.
And so the original question remains - will the monthly plan cost decline by the monthly device cost? By using the language above Telus can argue that it won't because they wrap it as a cost to upgrade or renew rather than the remaining cost to buyout the loan. But in the examples Telus gives at the link they also clearly state that the device cost is applied against the "discount" given at the beginning of the contract.
A comment I posted this morning, that isn't here now, might have helped you understand how a subsidy works. This is the second or third time my comments have disappeared. I'm not repeating myself if I can't be sure the effort of doing so will remain, so you'll have to go on with your misconception of the written material.