Is RE to CE an Upgrade?
No. Request RE used BMC ARS (Remedy) as it's database system. Request CE uses Cassandra as its database. Because of this shift, moving from Request RE to Request CE is a migration, not an upgrade.
Will I install CE on the same servers where I had RE installed?
While this is possible, it is unlikely. The requirements don't change for the web server side, but the requirements for the database side are now for Cassandra rather than the MSSQL or Oracle your BMC ARS/Remedy is likely installed upon. Also, you may be using your Remedy system for ITSM or some other use and wouldn't want to delete that or add a completely different new database on the same system.
Can I migrate any RE data direct to CE?
Request CE has a completely different logical approach to insert/remove. In CE, you write a visibility condition for a field that gets automatically evaluated when anything on the form changes. In RE, you would need to create an event for each thing that you wanted to have affect that field. This is the reason events are not transitioned.
Do my task trees need to be recreated?
Request CE is intended to work with Task 4. If you are currently using Task 3, you will also need to transition from Task 3 to Task 4. If you are already using Task 4, you do not need to recreate your trees. You will be able to set up webhooks for your existing trees--BUT anywhere you were using a handler to create, update, or complete a Kinetic Request item, you will need to update the trees to use a Request CE handler instead of the current Request RE handlers.
Will I be able to reuse my theme as is?
No, you will need to recreate your theme. We have had great success matching the look and feel of existing themes where requested/required, but the code will need to be recreated to account for Request CE's differences.