Hi, 
If I buy Sonar Professional with one time pay, can I later upgrade to Sonar Platinum later with monthly payment? If yes, how much will I have to pay monthly then?
 
Thanks!

Hi, and welcome to this forum.
Yes this is possible. Prices will change after May 1st 2017, so after that, no one (perhaps except Cakewalk) knows. 
http://forum.cakewalk.com/Cakewalk-Announces-Simpler-SONAR-Prices-m3573984.aspx
Hope it helps.
All the best.
 
 

IF you own Sonar Professional and want to go to Platinum monthly, I would imagine that the monthly cost for platinum would be the full cost of Platinum monthly minus the cost of Professional monthly.

Currently, this would be $US49.99 minus $US19.99, which comes to $US30.00 per month

Of course, the definitive way to know for sure is to log on to your Cakewalk account with the same email you used to purchase Professional, Go to the store and hit the "Buy now" link for Professional monthly.  You will then be shown your applicable price.