change the user group

You misunderstood me. I'm not talking about your demo site. On my site, I'm trying to make a change to the access rights of a user who fills in a specific field in the profile and updates the profile

Here's what I did:

Created a Yes and No combo box.
The user selects Yes and should automatically change the access level
I selected trigger "onafterfieldrow", user access level "Registered", type " Uesrgroup"
In the second tab " Conditions "I specified the field name [cb_demo], operator" Equal "and value"Yes".
In the third tab I specified to change the user group from" Registered "to"Demo group".
As a result of activation of the trigger (before changing a record in a field) on my website in the personal account of the user 504 error. The same error is in the administrative panel of this user.

I would be grateful if you could explain my mistake.

To change the usergroup based off the value of a field you'll need to check for the after update or after registration triggers where you want this to be applied. For example the below will work for frontend and backend registration and profile update.

Global
Triggers: onAfterUserRegistration, onAfterNewUser, onAfterUpdateUser, onAfterUserUpdate
Type: Usergroup
User: Automatic
Access: Everybody
Conditions
1: [var1_cb_demo] Not Equal To [var3_cb_demo]
2: [cb_demo] Equal To 1

With the conditions the first one is checking if the field value changed from what it previously was otherwise it would change their usergroup constantly when updating their profile when the checkbox field is checked. As for conditioning against a checkbox field to see if they've checked it you need to condition against a value of 1 or 0|Empty. 1 being checked and 0|Empty being unchecked (it's more reliable to use Equal To 1 or Not Equal To 1).