The spirit of this post is to gather other people's solutions, discuss ways of improving the suggested solution, and track future ArcGIS capabilities as they evolve for this problem.
This all started because we had existing data in a GDB that contained a Relationship Class (RC) between a Feature Class (FC) and a Table (TBL) using the FC's GlobalID field. We wanted to move that data into a new GDB that had a new schema (changes in domains, fields, etc.).
The problem is that when you Append the old data into the new schema, the GlobalID in the new schema is different than the records from the old data. This breaks the relationship between the FC and the related TBL.
I've got some bad news and I've got some good news. And some ugly details. But I also have steps/options.
The BAD news
Basically, ArcGIS Pro no longer allows you to change the datatype for step #8 in the solution below. This breaks the solution I originally provided.
The GOOD news
ArcGIS Pro 3.x now respects the "Preserve GlobalID" environment option in the Append tool, but with very specific parameters. It works if the target database is either Enterprise Geodatabases and Mobile Geodatabases, but not File Geodatabases.
The UGLY details
In order for the Append tool to actually preserve the GlobalID from your source to your target, you still need to check the "Preserve GlobalID" environment option in the Append tool.
You can only get it to work with Enterprise Geodatabases (EGDB) and Mobile Geodatabases (MGDB) as your targets, as no longer works with File Geodatabases (FGDB). Here's why.
The target GlobalID field must have and index that is configured as "unique". You can't make a GlobalID field that has a unique index in a FGDB.
Unique and ascending indexes are not supported for shapefiles or file geodatabases. These parameters are ignored when the tool is executed on a shapefile or file geodatabase data.
A 'unique' index can only happen if:
When you follow those two parameters, the index that gets automatically created for the GlobalID field that is has the unique setting.
If your target field is a GUID datatype, it must also have a "unique index". You must use add the index in ArcGIS Pro, on an EGDB table/feature class, and make sure the "unique" checkbox is selected before you run the Add Attribute Index tool.
The STEPS for Enterprise Geodatabases/Mobile Geodatabases
Below I only refer to EGDB, but if you're using a MGDB the same steps apply.
Once your data is in the updated schema, you can copy/paste it over to a File Geodatabase if you need to (for a deliverable or something).
The STEPS for File Geodatabases
All hope is not lost for users who must be restricted to File Geodatabase only. (But really, there is no reason to be afraid of the Mobile Geodatabase as a in between step, it works well!).
Thanks to user @DirtDogRoj in his excellently documented reply you can follow clearly outlined steps to achieve the same result. I think you could put it into Model Builder even as all the steps very systematic. You might like this method better just because! 😉
Pay no attention 2:
UPDATE 1: I have a much, MUCH, better solution that I discovered and it doesn't appear to be documented anywhere. All the text below that has strikethrough you shouldn't pay attention to, as it was the old, convoluted solution. New Solution: Pay no attention 1: After doing some searching, I discovered that there is a way to do this using and Enterprise Geodatabase (EGDB) and ArcGIS Pro. I post my basic workflow here on how to preserve the FC's GlobalID values so that when you migrate the data over to the new schema, the GlobalID values stay the same in the new FC. This preserves the relationship between the FC and the TBL in the new schema. Assumptions: Manual Steps: At this point, you can now copy/paste that new FC back to your location of choice, and rebuild the RC so that it connects up with the new TBL. It turns out that the GUID's used in the related table to relate back to the FC are naturally preserved by using the Append tool in ArcCatalog, so performing the workflow above on the related TBL is unnecessary. Even though the TBL's GlobalID (not GUID) values change when moving the data, that's doesn't matter to us because they aren't used to create the relationship. We don't do this often so we aren't going to take efforts to automate it, but I assume that might be possible.
Another idea is memorialize the GlobalIDs in the Feature Class (create new field called [GBL_GUID], GUID type --> Field Calculate the [GBL_GUID] field to the values in GlobalID). Now, if you export any data from that Feature Class in whatever database format.. the relationship will be maintained through the new [GBL_GUID] field.
After further testing, Mitch's solution won't work for us. The GUID field created doesn't autopopulate with a GUID in either ArcMap nor ArcGIS Online once deployed. Unless there is another trick to it, I'll have to stick to my original solution.
Will since you have an EGDB.....this is another one of those "Tech support will disown you" hacks #if they catch you doing it but...
In SQL Management Studio, change the object_flags to 4 (from 256) and sde_type to 14 (from 12) for the GlobalID column in SDE_column_registry for both feature classes. Then do you move. Remember to change them back!
very handy! I wonder if this can be mixed with some other python tools for archiving and downloading between AGOL and on-premise databases to achieve a true "two-way" sync without having to rely on flaky portal or pay for a data interop license.