Considerations while moving Customer insight Journey Components

1. The Marketing Email templates, Emails, Content blocks can be added as solution components , however, if there's continuous updates made to the Emails/some Emails need to be deleted in the Target environment,  one might face circular dependency between Emails and content blocks.

Keep the Email templates, Emails, Content block out of solution and move them with Configuration Migration Tool

With this, they can be dealt with as data and can be deleted/Updated easily.

2. Journey import - A journey cannot be imported to an environment, if another version already exists in.

solution:

-The Journeys Should always imported with just the Journey Name and the Json. 

Including any other columns might cause the Publishing to fail.

3. Segment Import - A Segment already referred to in a Journey should not be updated.

The Journey referring to it should be Stopped, deleted, 
Segment should be deleted and re-imported.

Then the dependent Journey shall be re-imported.

Upgrade Considerations (On Prem - On Prem)

While assessing Upgrade of System, Key considerations:

Unsupported Code - Of JS and C# -
For On premise, can use a Code to check for references to the unsupported code.

 (For Online, can use the Power-apps solution checker)

Reference to the OOB internal tables/fields that are meant to change - Example - String Map and Object Type Code should be removed/updated to use the recommended ways of the Table Name and Option set instead refer

Reports - Use Shared Data Source, to minimize repetitive steps during deployment

Number Of fields exceeding the maximum allowed - while importing solution from source
(older version)
to the upgrade instance (Newer version) - happens as newer version might have an additional set of system fields 

Ensure there's no Unsupported Customization:
https://learn.microsoft.com/en-us/previous-versions/dynamicscrm-2016/developers-guide/gg328350(v=crm.8)?redirectedfrom=MSDN#Unsupported