This information is covered in the link in the Stickeys expressed as simplified pricing. But for the many owners of previous versions some of whom do not read all the Stickeys, it is worth noting that if you plan to wait and buy a new version of SONAR expecting a big discount as has been the custom with Cakewalk that is not going to be an option for long. For example any owner of Cakewalk Producer can currently get Platinum for about $200.00. After May 1  they will have to pay full price, whatever that is in the future. Currently it is about $500.00.
 
http://forum.cakewalk.com/FindPost/3574019
 

also, you need to add it to your basket to see your discount (even if you are logged in)

It's worth noting that the "big discount" in the past was for upgrading from the previous version, and the discount quickly reduced for older version upgrades.
 
With the introduction of continuous updates the "previous version" is now several years old and would normally not be eligible for such a large discount.

According to their blog there will still be upgrade discounts: http://blog.cakewalk.com/cakewalk-announces-simpler-sonar-prices/.
 But perhaps not just to get the newest version of the same "level" (old platinum to new platinum).

"Upgrading SONAR to a more advanced edition

Starting May 1, 2017, if you want to upgrade your copy of SONAR to a more advanced edition, you simply pay the difference in the current price.
If you own SONAR Home Studio, upgrade to SONAR Artist, Professional or Platinum for $49.99 off the regular price.
If you own SONAR Artist, upgrade to SONAR Professional or Platinum for $99.99 off the regular price.
If you own SONAR Professional, upgrade to SONAR Platinum for $199.99 off the regular price.
We will no longer offer discounts for any other editions of SONAR or from other Cakewalk products."

Yes, the term upgrade is ambiguous, and the term previous version, is still mucked up if people are thinking in terms of "continuous upgrades to the current version." What is clear is that anything older than X3 as a version name will not qualify for an upgrade at a discount to the purchase of the now current version of SONAR. 
 
What you are seeing is a discount offered move up to a more expensive version name of SONAR (the current version).  Presumably, this payment will unlock and download all of the features of the then current version that you will then be able to use in perpetuity if you have completed the full purchase. For monthly payers, it is not clear that a $49.00 payment at month 10 of your monthly SONAR Artist membership to convert to Home Studio will be as valuable to you as the same payment at month 2 of the membership, and if you never complete the 12 month rent to own minimum, you could end up paying a lot for a short period of access to increased features. In other words, lacking more clarification, you may pay more for less the longer you wait to upgrade during your monthly payment plan as the upgrade to a more expensive version will not be prorated or charged as 1/12 of the $49.00 per month upgrade price in the Artist to Home Studio example. 
 
It does seem clear from the rest of the policy, that the upgrade to a more expensive version during a current membership will entitle you to renew the membership to the higher priced version, but the price you pay will will apparently be the "no more than 50%" of the then current price for that more expensive version. It is not absolutely clear what the downgrade pricing would be if you wanted to renew a Platinum membership with an Artist membership for example, although it looks like that might still be charged at the less than 50% price of a new Artist ownership.

I could be wrong but (a few months ago) we were all asking ourselves how will cakewalk survive if it is giving away lifetime upgrades. Well it appears they have a marketing strategy in place to move them forward. Its nice to see them taking a course (navigating wise) and planning for the future.
We all want cakewalk to be around for years to come.

Sonar is like a lot of Software where you need to pay attention and watch for deals. 
Don't complain if you wait to long and miss the boat. 
I almost didn't take the bait for lifetime thinking I don't really need anything more, but I think all of us will be smug when we look back at that deal when we hit the fall of 2019 and we still haven't had to reach for our credit cards. 
But just wait and see, they will hit us up for fancy add on's, say things like Drum Replacer will be extra $$. down the road. I think they made this clear to us when they gave us the lowdown on lifetime. 
Our Lifetime updates will be mostly bug fixes and small changes but that's still a bargain. 

BRB, getting popcorn.

This is JMO. I too read it as "before X1" will get no discount on an upgrade (IOW there isn't one).
 
With no disrespect to those still on 8.5 or earlier I would say they are no longer a loyal cakewalk customer. Loyal being the operative word here. 
 
As we are wont to do here, let me use an auto analogy.
 
My 2005 Honda does not qualify me (IMO) as loyal Honda customer if that is the only Honda I own. I am a past customer, but my 2005 does not get me any discount if I went to the dealer and bought a 2017 vehicle (of course I might choose to trade in or sell the 2005 on the street, but that does not apply to the Cakewalk product model, i.e. software).
 
Again, JMO as to how I read it. I'm not saying I'm right.
 

Cactus Music
 
Our Lifetime updates will be mostly bug fixes and small changes but that's still a bargain. 

If the lifetime always keeps me compatible with the latest Windows version, I will consider it a win!