The old version v0.0.4 is still running perfectly stable. So is it necessary to upgrade to the latest version as well?
yes, you don´t get rewards until you update. just be sure to delete c:\user\yourusername\ .silicoin folder.
Yes, you should upgrade. The old versions run on the old chain and you will NOT get rewards using the old version.
Yup you have to. The updates v0.0.9 of Silicoin is great. It fixed the high filters problem and support the new chia plots already. The first chia fork who merged the new codes from chia and mine with NFT plots. You should try it man, really.