Hi John
I've tried resetting and rescanning several times but no joy. I suppose I will contact support and see what they say about it

Update! Cakewalk has told me there is a known migration problem from X3 to Platinum which is being worked on and a fix should be available in the next week or two. In the interim they have suggested I go to Command Center and reinstall Platinum as 32-bit. I have done this and now all of my Pro-Channel Modules (including ReMatrix) are once again working and Melodyne now also shows back up in the region fx. I also had to re-install Bark of Dog as 32-bit, which is now available in my list of modules and working correctly. Until this next update comes out, it appears I am stuck working in 32-bit.

I wonder what the specific migration problem is as many are successful.

Yeah not certain. Here's what Cakewalk wrote:
 
"There is still a known issue with plug-ins when migrating from X3 to Platinum, and is still being worked on. For the time being, you should be able to work around the issue by installing and running the 32-bit version of SONAR. This can coexist with the 64-bit version with no issues. You can find the installer in C:\ProgramData\Cakewalk\Command Center\Downloads. I apologize for any inconvenience this may cause". 
 
I am happy to at least be able to use Platinum now, even if it's only as 32-bit!