When designing my new blueprints (updating old ones) for color coded pipes, I regularly run into the following issue:

While building, the old regular pipes and storage tanks are not filled with fluids, so I can not use the planner tool.
What I do is update them by hand for the blueprint. For that, I must tear down all the entities and build new ones, if they had logic network wiring, I need to redo that as well.

For this, it would be extremely helpful if I could build the entities on top of each other, as in upgrading them. (e.g. place a water tank on top of a regular tank, just like you can build an assembler 2 over an assembler 1.)

I'm worried I'm possibly misunderstanding your message, because that is currently possible in the 1.1.0 release. If you place a blue storage tank on top of a base storage tank, it is able to be fast-replaced and keep all the circuit wire connections and even the fluid inside it. The other way works as well, "upgrading" a color-coded pipe with a base pipe. It uses the same logic as assembler tiers, where all the pipes that should upgrade eachother share a fast_replaceable_group.
I'll leave this open for now, as I haven't been able to replicate the issue. If it continues, please let me know if you have any steps to reproduce it and I will look into it again. Thanks :)

I will test again. That's exactly what I meant. Maybe me recognizing it and writing it here was just around (before and after) your release of 1.1.0.

Hi again, and sorry for needing so long to get back to you.

I was working on fixing another mod, and to not keep the author waiting too long for a merge/pull request ... yada yada, I was under time constraints, and when loading your updated mod, it loaded the broken changes, took me a day to get my game back to not crash anymore.

I did give it a try now, and it works just as expected, thanks for this change and sorry to bother you!

Keep up your good work.
Cheers!

No problem at all, I'm glad to hear it's resolved :)