Am I correct to assume that this means I get no more updates? (last one with regular Sonar Platinum was October 2017).
Yes, Gibson shut down Cakewalk operations (including SONAR development) in Nov 2017 and sold Cakewalk IP to BandLab. In April 2018, Bandlab released a new product called Cakewalk by BandLab, a DAW based on the unreleased SONAR Platinum 2017-11,
I'm concerned that I have to DOWNLOAD (!?!) Cakewalk by Bandlab, when I already have Sonar Platinum. I don't want to risk losing plug-ins, functionality, or any existing projects.
If it's a seamless transition with no risk of any loss of any functions, that would be one thing. But If I have to download a new version of something I already have, that concerns me.
If Platinum is on the PC, after upgrading to CbB - all the plug-ins bundled with Platinum, all the functionality of Platinum and all existing projects will be available in CbB. In fact, both Platinum and CbB will be available on the PC. CbB installs as an upgrade to Platinum because....it is an upgrade. To see the fixes and features made to CbB since SONAR development stopped read this