It is no longer possible to downgrade iOS 10.2.1 to iOS 10.2 as Apple has just closed the signing window for iOS 10.2 software release. This means that users can no longer update, downgrade, or restore to that firmware, either using over-the-air mechanism or through iTunes. It is also impossible to save blobs for iOS 10.2 through any tool.
Yalu 10.2 jailbreak that was released last week supports only for 10.2 firmware version. This means that 64-bit iOS devices owners who have already upgraded to iOS 10.2.1 will not be able to restore their device to iOS 10.2 to jailbreak it. Those who had already saved the required SHSH blobs in time are really lucky. And if you who have these blobs safe but did not restore to iOS 10.2 in time you can now rest relatively easy, knowing that in theory tihmstar’s tool Prometheus will allow them to upgrade to iOS 10.2 at a later date. But if you have not already saved the SHSH blobs for iOS 10.2 for your device, it is impossible to do it now as Apple has stopped signing the firmware. Since Yalu 10.2 jailbreak doesn't support iPhone 7 and iPhone 7 Plus, owners of these phones have very little reason to downgrade to iOS 10.2.
Folks who have already updated to iOS 10.2 and has already jailbroken it are lucky! Be cautious with your jailbreak now, and careful with the tweaks you install, as not all of them were updated to this version. A little mistake now could mean a restore to iOS 10.2.1.
iOS 10.2.1 is a relatively minor update when compared to iOS 10.2 release and only brought some security patches, bug fixes and any new features.
Few days back Apple has already seeded the first beta of iOS 10.3. This update comes with a few new features such as Find My AirPods, user security section in Settings, APFS, and more. The final version of the OS is expected to be launched in March.