How to change Power Query source and keep all steps and queries etc

Hi BenBlackswan ,I am glad to help you.
Unfortunately, if you have more than one data source, you will need to modify each one of them.
If what I provided above about modifying data sources in power query works, for your second question: if your report relies on several different data sources, you could modify them one by one in the query editor of power query. You could also create parameters and use them instead of complex data sources.

Here are my test results.

The advantage of creating parameters is that you can centralise the management of parameters, and in the future if your own data source changes, you only need to change the value of the centrally managed parameter, which saves management time. However, a parameter generally represents a path to a data source, so you will need to create multiple parameters to manage.
I hope my suggestions give you good ideas, if you have any more questions, please clarify in a follow-up reply.
Best Regards,
Carson Jian,
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