Hi ,
>>can i upgrade the PC-B to Windows 10 1909, will it resolve the insecure issue?
No.
In your case, the main reason is because Guest account access to a remote server is disabled in windows 10 1709 and later version.
If you are using guest account on windows 10 and cannot enable insecure Guest logons by registry or GPO, then whatever PC-B's OS version is windows 10 or not, you still cannot resolve the insecure issue.
As you can see from the following article :
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Best Regards,
Candy