Created: 5 Mar 2021, last update: 30 Jan 2022
My experience with Upgrading to Sitecore 10.1
Upgrading has changed in Sitecore 10.1, it has become simpler.
Improved Upgrade Experience by storing default content items outside of the database, removing the need for applying Update packages.
The documentation now describes installing a new instance and update the database. Instead of updating the existing environment. This is also more how you do it in practice, as an upgrade often has a lead time of several weeks and you want to keep your DTAP street over time. In my case I upgrade a Sitecore 10.0 to 10.1. I may start with an empty experience database. That will save me a lot of time.
These are the steps in the upgrade guide 10.1:
- Upgrade your databases to ensure that they are compatible with the new version of Sitecore XP.
- Install the new version of Sitecore XP beside your existing Sitecore XP installation.
- Attach the databases to the new installation.
- Deploy your solution on top of the new installation.
- Perform some post-upgrade steps.
Instead of upgrading and Attach the database, I choice for backup the old database, the master,core and experience forms. And overwrite the new 10.1 with this one. After that I run the upgrade SQL script in my case just 1 SQL script.
Running the new Sitecore.UpdateApp.exe clean tool
This was done in a few seconds for my 4GB big database. (Also 4 items in the web database that was a clean 10.1 database, should be zero. with a publish I get my web database up-to-date, but a publish cost me probably more time than just upgrading, anyway) 
If I compare my master database, it is about 30 MB smaller and core db is 58 MB smaller. The upgrade experience was very good compared to the old way of running upgrade packages. I never run the UpdateCenter in production it was complex with all steps in the old upgrade manual, specifically a Scaled environment, spinning up a new environment is easier.
Conclusion
It is now very easy to get an updated master, core and web database.
If you are in doubt between updating to 10 or 10.1… upgrading to Sitecore 10.1 makes it a lot easier for you.
Sitecore Fast Query is no deprecate, it does a database query, and since not all items are in the sql database now you can get a different result than in pre Sitecore 10.1 versions
Stay tuned, Dev upgrade is done if I have any more findings, read it here.
