How to transfer photos from internal storage to SD without impacting Google Photos and Dropbox?
Note: this answer is based of experimentation and some speculations at the end.

I tested with a few pictures, and then with a batch move of the pictures from phone storage to the SD card.

The result was OK: the pictures online did not change, there was no upload despite pictures appearing on the SD card.

I assume therefore that the phone does some housekeeping and keeps a trace of what was uploaded (so that it is not uploaded again)