Will the update affect a normal user and standard EOS functions? Users will be able to use your account as normal. No changes or actions are required from the normal public users of EOS.

Will there be an interruption or downtime on the EOS mainnet? No downtime is expected with the update as long as it is executed as expected. The hardfork has been tested on multiple testnets and they have all progressed without any downtime.

Will the update affect my current smart contract deployment? No, but you can benefit from it's features to make your dApp easier to use for the end user. Like you can stake the RAM for the users to make your dApp UX much better. Please try and run your application on 1.8 to be sure that the upgrade does not affect your dapps.