UPDATE: I've written a blog post descibing how to do this, along with some additional tips and tricks. See here:
https://helpfulbit.com/upgrade-flows-to-use-connection-references/
Hi. As far as I can see, this question has not been answered to @dave8 's satisfaction.
My findings: If a flow is created manually outside of a solution, it uses connections. If it is then added to a solution, it still uses connections. I cannot find a way to change this.
If a solution flow is copied, it has to be manually re-added to the solution, and uses connections.
This is very frustrating as it can cause dependency hell trying to import it to the target environment.
I have not yet found any way to convert a flow using connections to use connection references instead.
So far the only work-around I can see is to create a brand new flow and, in the case of copying, use the clip-board feature to copy the steps manually from the original flow.
Granted, connection references are still a preview feature, but if MSFT is watching the thread, these are two separate items that could greatly improve the UX:
1) The ability to copy a flow within a solution, with connection references intact.
https://powerusers.microsoft.com/t5/Power-Automate-Ideas/Duplicate-copy-a-flow-within-a-Solution/idi-p/752370
2) The ability to update flows to replace connections with connection references.
https://powerusers.microsoft.com/t5/Power-Automate-Ideas/Replace-Connections-in-Flow-with-Connection-References/idi-p/1229312#M26136 (Newly added).