How to start with GWT upgrade from version 2.1.0 to 2.9.0, (java 1.6 to java 1.8)

Hi Team,

(Apologies if the answers to my question are based on individuals opinion based, but i am in dire need of your help)

I am currently stuck with an old GWT 2.1.0 application written in Java 1.6 which needs to be updated to java 1.8, so we are planning to upgrade to latest GWT version 2.9.0. There are no changes to the application code or any new enhancement request in last 5 years, considering this our client is expecting no changes to look and feel of the application but from the compliance perspective we are required to upgrade to at least Java 1.8.

But i have no experience and never used GWT, and there are very few guides out there on upgrade from 2.1.0 to 2.9.0 (I know its a big jump). Can someone help me understand, how to start the upgrade process, and how much code changes would i need to make the application compatible with the new version, will it be complete re-wright of the UI component of the application? or will it be minor changes to existing code ? Finally, if the upgrade requires complete re writing of UI component, is it worth it for me to lean GWT (considering recent slow down in upgrades to GWT) and uses it in the upgrade instead of complete rewrite of UI in React. 

Thanks in advance. 

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